Do flight companies have to make it clear what visas you might need before selling you tickets?

You expect a higher level of customer service.

You have in your mind a certain level of attentiveness and customer service. You want your provider to pay attention to details like that, and work with you to help your travel be a success.

That’s fine and a reasonable thing to expect. From a quality, full-service travel agent.

And I remember when that was pretty much mandatory. You didn’t get on a plane without a machine-printed ticket exactly the size of an IBM card, with perforations and break-off sections, that came off a very expensive printer that the travel agent leased, and tied to a 3278 terminal on a special phone line. As telephone call centers, automated phone systems and the Internet changed all that, legislators decided consumers should be free to choose discount providers. Which works for me.

But now that such discount providers have become normalized, people don’t realize a full-service agent is even a thing, or a thing for them because they prefer "cheaper". True; agents don’t work for free.

When you book travel yourself on the Internet, you’re the agent, and it’s your job to educate your client on travel issues like visas. That’s why you get the big bucks! 🙂 Or rather, keep the big bucks in your pocket.

So you chose to DIY the visa stuff when you decided to deal discount/direct. That was your choice and they don’t owe you anything.

It is strictly impossible for an airline to know for sure and in advance if you will be allowed into a country or even allowed to board.

While the general cases can be relatively easy, there are so many special cases and exceptions it’s not even funny, there’s a lot of information they don’t even have (like previous travel history), and authorities have discretionary power to reject anyone at the border, or for countries with bidirectional API (like the US), at time of boarding (the dreaded “DO NOT BOARD”).

Even without getting to that point, tools like Timatic have a few limited cases where they will say “no visa required” or “visa required”, and then tons of cases where they have long lists of conditions and exceptions and special cases which are not standardised enough that you can just enter data and get a verdict, you just get something a human must interpret (which is often not quite obvious), sometimes pages and pages long, and is sometimes subject to errors.

Add to that the fact that things change quite often, and you get a big mess, which airlines absolutely do not want to enter, as it could open them to a whole lot of liability. So they just stay clear and put the responsibility squarely on the shoulders of the passenger, and they honestly very little other choice.

Many airlines and OTAs will make it quite obvious and explicit at the time of purchase, but it is always spelled out in the terms and conditions.

Questions like that, or complains about specific cases where it did not happen, are common on this site. But think about what you are asking the ticket seller to do:

They would have to be aware of the entire travel and legal history of their customer, as well as all citizenships. A years-old overstay might mean that the customer cannot get a visa on arrival, and has to apply beforehand. A second or third citizenship might mean that the customer needs no visa. And even then, the customer might be on a secret no-fly list held by some country.

Or they hand out a pages-long questionnaire, with lots of questions that are either insulting ("have you ever been convicted of a crime of moral turpitude") or impossible to answer ("do you have the same birthdate and shoe size as a wanted terrorist?"). Then they feed that into a computer system and tell the customer the answer.

For easy cases, it would actually be an option to require that from any ticket seller (travel agent, tour operator, …). But it would also mean that people with complex cases would not be asked to think through their specific situation.

No, flight selling companies tell you that you need to look for the needed visa yourself.
That is airlines as well as sites that only sell the tickets.

There are even sites that sell a lot of unconnected flights, with very short layovers, where most people need visa, without a clear warning about this.

With the warning that it is on the customer to make sure they have the needed visa they cover the legal aspect. (But I think in some countries you might be able to argue that they should have done more and you may get a partial refund, but I have never heard of that happening.)