Is hidding or cheating about ones identity unskilful and probably harmful for others as well?

I would suggest the simple answer is: since hiding or cheating falls under violating the Fourth Precept, it is harmful. Since everything is interdependent, it is harmful to others.

However, we can also consider another perspective: none of us truly knows him/her self. We constantly wear social masks and believe our social identities and roles define us, and we like to manifest this positive self-image toward others. This is why according to the Kalama Sutra the Buddha said:

Do not believe anything merely on the authority of your teachers and priests.

Because they are subject to that same desire to manifest their projection of identity and authority.

In letting go of this desire to define ourselves is the realization of the illusion of self and other. The question drops away on its own.

But don't take my word for it ;)

Anonymous (online) Bikkhus

This topic was about whether someone was at fault when they said something (asked a question) to someone online, assuming that other person was a lay-person, if that other person was actually ordained but not advertising that status in their online persona.

• I don't feel comfortable passing judgement on how a monk may use the internet. Instead, doesn't a monk have a preceptor, isn't this kind of question (discipline) something a monk or a nun may discuss with their peers?
• Assuming people don't lie, the software (the web site) need not to do something special (e.g. different colours for user names) -- instead the user name or avatar or user profile information is sufficient (see e.g. Tenzin Dorje, yuttadhammo, and other users e.g. with "bhikkhu" or "bhikkhuni" in their names).
• I'd tend to agree with one of the comments there, that if a Bhikkhu doesn't appear to be a Bhikkhu, and if someone addresses them as if they weren't a bhikkhu, then the person addressing them is not wrong-doing -- because if they don't know that the one who they're addressing is a bhikkhu then they can't have the intention to mis-speak

• Perhaps though you shouldn't speak one way to a bhikkhu and another way to a lay-person, for example:

  • Perhaps you should try to "be nice" (and not disrespectful) to everyone
  • The question Who here is a Sotāpanna, Sakadāgāmi, Anāgāmi, or Arahant? was closed on this site

• I can't tell whether the OP is being sarcastic, and for what reason, when he or she starts to address as "Bhante" someone who claims not to be monk, towards the end -- predictably, "sarcasm" is mentioned in an essay on right speech

  For many of us, the most difficult part of practicing right speech lies in how we express our sense of humor. Especially here in America, we're used to getting laughs with exaggeration, sarcasm, group stereotypes, and pure silliness — all classic examples of wrong speech. If people get used to these sorts of careless humor, they stop listening carefully to what we say. In this way, we cheapen our own discourse.

Cunda Sutta: To Cunda (Snp 1.5)

I'm not sure how that's related to the question. I guess it's the verse,

Creating a counterfeit
of those with good practices,
self-asserting, a corrupter of families,[1] intrusive,
deceitful, unrestrained, chaff,
going around in disguise:
he's one who corrupts the path.

The last verse ...

Any householder, having ferreted these out
— a discerning disciple of those who are noble —
knowing they aren't all the same,
seeing this, his conviction's not harmed.
For how could the corrupt with the un-
corrupt,
the impure with the pure,
be put on a par?"

... suggests that any householder can and should be able to discern the difference, doesn't it?

I think that regular users of an online site probably do form a (private) impression of other users -- e.g. who is self-asserting and unrestrained, who isn't -- no matter by what name the other users identify themselves.

Perhaps people should beware though that comparing people can be considered 'conceit'.

Iti 25

For the person who transgresses in one thing, I tell you, there is no evil deed that is not to be done. Which one thing? This: telling a deliberate lie.

That's in one of the suttas to Rahula too.

It might be worth noting that the vinaya's description of a lie includes both "intention to deceive" and "making the effort to tell". If you don't say something, I think that's not considered a lie.

Avatar and User placements

• As we all know, the internet is often used as a playground to test some things but also a beloved place to try to addopt a different personality.

  Yes, perhaps so. After I used the internet for a while (in the 1990s) I decided to use a user name (e.g. "ChrisW") which I could recognise as being my own name. I don't remember that I was ever cruel when I previously used a more anonymous pseudonym, but maybe laughed too much, too many (maybe silly) jokes, insufficiently serious.

  I don't consider that as "intention to deceive", though, because I expect that no-one will assume that's my real or legal name ("Chris" is a diminutive of my given name, and "W" an abbreviation of my family name -- this kind of abbreviation is one of several conventional ways to create an online user name).

  Incidentally, do you know why people are given new, Buddhist names (e.g. Ven. "Bodhi" and so on)?

• On this level it has nothing to do with pride

  Really? I assume that care to maintain one's reputation, maintain a good name, having a sense of shame, are all matters of pride, are they not, but perhaps used to a good end.

• One example is the identification with a picture that one likes to represent.

  I find it easier not to use/choose one, I hope that's alright.

  I've learned that in France it's conventional to include a photo of yourself with your resume in a job application -- which I found very very surprising because in 'my home country' you'd never include a photo. The problem with a photo is that it allows people to see your age, your gender, your ethnicity -- which are all (according to my home society's employment laws and conventions) invalid reasons for preferring one job applicant over another.

  In summary if you want to judge me I expect you'll do that based on what I write, not on my photograph.

• I guess I have mention it already on another place, that it is very important to declare ones gender, ones age and the status of practice as one protects others from doing wrong by wrong estimations and one also takes care of one self not to interpret (procects) one self into something one is not really.

  I assume you can estimate my gender, age, and status fairly accurately -- I make no effort to conceal them. And if I don't tell you and you estimate wrong, I don't think that's your wrong-doing.

  Perhaps everyone should generally be careful not to assume (take as true) things that they don't know, though.

• A monk for example, is ... also not allowed to teach if ones face is hidden.

  This site, this whole medium (i.e. online, text-based discourse) is what it is.

• At this point (in english its a little dificult for me to use old polite language)

  Yes. French (perhaps like German) still preserves e.g. a polite distinction between singular tu and plural vous (German du and sie if I recall correctly).

  Here (outside a city) it's normal to greet (say bonjour, "good day") to everyone you meet (e.g. when you're walking outside), and it's polite for the more junior person to greet the older person first (and I don't wear a hat but I take my sunglasses off when I do that, just to be polite, you know).

  I think that even in France (or in French) it's conventional to be less formal online -- for example to address people as tu on a discussion board.

• It is good and proper, to inform others about your practice status ... about your age and you gender.

  One of the things I've noticed about this site is that people don't use the chat room. When I was (previously) active on other Stack Exchange sites, the site's chat room was used -- to greet people, to discuss the site, to joke, to chat about people's everyday concerns.

  This site (all Stack Exchange sites) are especially for Q&A, so the content (i.e. the topic) is important and the identities of the users are less so. In fact, questions about users (e.g. "how many of you users are Theravada?") are explicitly off-topic.

  If you want to get to know people/users in other ways, if users want to chat with each other -- and I'm not saying they shouldn't -- then a chat room is the place to do that, for example this chat room.

  To be allowed to talk in chat, though, you need a reputation of (only) 20 on this site (I guess that policy helps to keep chat free of spammers). You could easily have a reputation of 20, but you'd have to be willing to "register" your user name, i.e. by replying to the email, which SE sends you when you create an "unregistered" account (instead of discarding the "reputation points" which you accrue here, by having a series of different unregistered accounts which are all named "Samana Johann", but which the system is unable to merge and which are discarded whenever you use a different computer).

  Your using unregistered accounts also means that people are much less able to find everything you previously posted on the site -- but actually there's no rule against it, so if you think that's for the best then you do that -- you may or may not want to reconsider that.