Kill switch in DJI drones and other brands: https://npo.nl/start/serie/konvooi/afleveringen/seizoen-1

Documentairy in Dutch but the interview is in English

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-...