Why doesn't my passport show if a foreign country has banned me?

Isn't that the point of passports? To contain data and violations on each traveler, and that data is shared between all countries who "access" or scan the passport to make a decision on whether to admit them or not?

The passport is just an identity document that has some stamps. A passport is not the right to enter a country. That would be called "citizenship" :)

It's also not really a data port. It's not like a team of technologists led by Elon Musk did a from-scratch reboot of passports using 2018 tech. The whole world has to agree to basic technology changes. It's been 2500 years of incremental improvements.

That said, each of the countries try to do their best with off-passport databases... and a few of them share data. EU obviously, but the Five Eyes and EU as well.

You apply for a visa

Thanks for the information, but there is still no resolution to the issue: How then does someone find out they are banned from a country before traveling, without attempting to cross that country's border?

You go ahead and apply for a visa to that country, in the normal fashion. That's it. They either grant you a visa or they do not.

"But I don't want to apply for a visa! My citizenship country has some sort of visa waiver with my destination country, and I'm able to do a visaless entry normally. I want to know if I broke that by violating their rules."

You already (should) know if you broke their rules

That's your answer. They usually publish their rules, so just compare "their rules" to "your behavior" and that should settle it.

"But I want to know if they FOUND OUT about it". Okay LOL. I hate to say the answer is usually "yeah, they know". But I gather you are not an immigration lawyer who specializes in that country, so you may not have ALL of your facts straight about whether what you did is the kind of thing they ban you for. There are 3 ways to find out.

• Talk to an immigration lawyer who does specialize in that country.

• Attempt another visa waiver entry and see what happens but make room on your credit card, as you may be about to pay full price for a same-day return airfare. Note that if a country does preclearance at a given airport, use that route! That way you avoid a miserable and wasted double flight, and may even be able to salvage a respectable holiday getaway out of it. (e.g. if in the EU going to USA, book through Shannon Ireland - make plans for both a USA holiday and a Shannon holiday.)

• (the surest, safest method) apply for a visa, as discussed. That also gives you the best option to explain your case for why the previous transgression should be overlooked, since you have the opportunity for a written and organized statement with unlimited supporting docs, not the hostile chaos of an immigration desk at an airport.

No, that is absolutely not the purpose of passports.

The purpose of a passport is to demonstrate that you have the identity and citizenship that you are asserting, and to use that to decide whether to admit you into a country or not. It is also a secure place to store visas that have been issued to you, and proof of when you arrived at (and in some cases, but not all, left) a country. Some countries may use that record in addition to your identity and citizenship to make decisions (not letting you in if you have a stamp from a specific other country) but this is the exception, not the rule. Others may use it to confirm that you are someone who enters countries and then leaves again as planned, increasing their confidence in letting you in. But this is not the purpose of the passport, merely a side effect some countries use in some cases.

Passports do not contain violations, except occasionally when a visa or entrance stamp will be "cancelled" such as by an officer drawing an X over them. Some governments keep lists of violations, and some share those lists with other countries, but it's not universal. When they do share, the mechanism they share with is not your passport, but another form of communication.

The scanning is a quicker way for the officer to enter the information needed to look you up in some system that the officer has access to. This will not be a giant international system shared by all the countries of the world, but something specific to that country and whatever lists it may have of people who have been banned or who are going to get extra scrutiny on each entry. There was a time when the officer typed those things in by hand, but now they can generally swipe instead, or you can put your passport on a scanner and do the whole thing without interacting with a person.