I am against city cultural flip because of enormous gameplay frustration/exploitability, and border cultural changes because of lack of realism.
I mean, borders on land, between established states, are a static political entity (by 'static' I mean, change only via conflicts or diplomatic treaties) and shouldn't be confused with 'cultural influence'.
Are there any historical examples of country borders changing without use of military, diplomacy or will of government just by cultural clash (???) of both neighboring empires causing settlements (here I mean not only cities in civ terms but all tiles, they're inhabited after all) switch between sides and suddenly become loyal to neighboring state? Without rebellion, conflict etc?
Probably yes, but it'd be rare cases of limited scale ethnic divisions (and I'd say government almost always used coercion in such situations), and we are talking about all empires of the world being able to casually take territory of all other alien empires not with military or diplomacy but, uhm, citizens (???) and that happening all the time across history.
Okay I can imagine Ucrainian city flipping to Russia, for example

but that's one of very rare instances when such mechanic would make sense.
For me it's complete arcade nonsense. Borders are borders and shouldn't be changed "by citizens" without governments agreeing via diplomacy or war. I disliked culture_affecting_borders n civ4, civ5 great general iteration and great artist iteration. It leads to weird situations, cheesy exploits (especially in MP) and breaks immersion.