What does it mean when someone's pronouns are "she/they" or "he/they"?

Yes, it’s there, but not in any way designated as a pronoun preference. The only indication we have that it might have ever been so designated is @Colibri saying…

apparently in her bio on Twitter she is still using “xo.”

… in reference to what pronoun she was using before “she/they”.

What I was saying is that I don’t know why Colibri thought “xo” was related to Halsey’s pronouns in any way.

Powers &8^]

I saw it mentioned in some other article. I never saw it myself in her Twitter bio. Sorry for any confusion. You can probably just drop this.