I asked for proof, not your personal anecdote. I too have lived in many cultures. Singular they has been commonly accepted everywhere that speaks English.

Think about the phrase "Everyone loves their mother". Everyone is a singular noun. According to you, this usage would be debated everywhere, because the "correct" English would dictate "Everyone loves his mother".

I dare you to find an English speaking country anywhere in the world where the majority thinks "Everyone loves his mother" sounds more correct. You won't. Because singular they has been a thing for centuries and is entirely common at this point.

The part where you're tripping up is how singular they is used. It is common everywhere in the world to use it in situations of ambiguous identity. The reason you find it absurd is that now we are using it in situations of specific identity but ambiguous gender. That form of usage is indeed new and not majority usage everywhere, but that still does not mean it is incorrect. Actual communication with people is what defines correctness is the English language.