Everyone has “custom” pronouns. Some just match possible preconceived notions about them.

Specifically on she/they:

I use this on the internet sometimes because, when I give someone help on something technical, 90% of the time they thank an imaginary bloke. And if I correct them I’ll get trolls harassing me and patronising pricks giving me tips for beginners (including, often, the person whose question I answered).

When you don’t know, keep it neutral. Reinforcing stereotypes is harmful even if you mean no harm.