I think being able to flush paper is the exception, not the rule in Europe, particularly the further east or out of a touristy areas you go. The older sections of cities have some really ancient plumbing systems.
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I haven't been everywhere in Europe, of course, but I really didn't run into any "don't flush TP" things. Even in those really old buildings I'm Rome.
That's not exactly uncommon around the world.
No, it's a "our plumbing isn't up to American standards" thing. Worked in a place with some goddamn Haitians that couldn't get it in their heads that we flush the TP here and they instead used the garbage can. It was a bit annoying seeing their shit-stained TP in the trash.