I couldn't find a source for China disabling drones it sold to Ukraine, but they did cut off drone exports to Ukraine, while still supplying them to Russia:

https://www.newsweek.com/china-ukraine-russia-war-drone-uav-...

1) I have heard of the kill switches being used from several sources in the Ukraine ministry of defense. I advised them on how to remove the kill switches in two brands.

2) It was mentioned in an interview of military in this Dutch documentary https://npo.nl/start/serie/konvooi/afleveringen/seizoen-1

3) Several journalists mentioned it in their news reporting (sorry, I can't locate the links right now).

4) Two soldiers vlogged about it on youtube.