Well, how do you expect a website to know that you are 18+?

The technical solution offered earlier was to have a trusted third-party only be willing to answer the "is this person 18+?" question. Of course, the same works for banks. The trusted third-party can answer "does this person own this account?" without needing to reveal to the bank any other information about you.

Now, that still leaves open the question of who you can trust. If you can trust a business run by people then that allows you to trust a bank, sure, but it also allows you to trust a tech company, so why not just give the tech company your ID and skip all that technical complexity? Understandably the broader idea presented earlier was that you cannot trust businesses operated by people, but then that includes banks, so...

For the sake of discussion, if we accept that technical solution and the need for a trusted third-party that is a business run by people, surely it should at least be a business that does nothing but offer profile trust to minimize the blast radius? For what reason would we want that business to have their hand in other activities like chat or banking?