Did the underlying bits (hex/oct/… or whatever representation) actually change or just the visuals?

Passwords are more secure if they are higher entropy, so it makes sense to support a larger variety of characters, Czech or emoji.

It seems paramount that the OS should not allow password input of any characters which it theater takes away. At the very minimum if this is absolutely necessary to make this breaking change, the user should be warned several times that a character in the password is no longer valid and maybe even prevent the OS from upgrading before the password is changed to a forward-compatible one.

I think there's a distinction to be made between 'is it a good idea for someone informed enough to know how these things go in the real world?' i.e. the HN audience and 'should this be a real worry in a sane world?' to which I say no, it shouldn't be a worry that if I was allowed to enter a password today I may not be able to tomorrow.

That's just excuses for moronic decisions of trillion dollar companies.

In my password, I have the Collectivity of Saint Martin flag emoji and United States Minor Outlying Islands flag emoji next to the French flag emoji and US flag emoji. For good measure, also the flag of Chad next to the flag of Romania. I am sure it's not going to cause any issues.

Maybe.

But there is already a known pattern on how to handle this which I was taught (before the original iPhone even) in university CS studies:

If the manner of entering credentials has to change,

Then on first entry, offer the old method,

And, because you now (temporarily) have the plaintext credentials, you can now inspect it and test if anything need to change for the future,

And then set a flag, or require user action , or just re-encode, to use the new method as inspection determines.