It’s the Russians that keep making ultimatums hinting at use of nuclear weapons. Hasn’t happened yet, fortunately.
Ukraine is doing a ratty good job of defending their country. If they receive too much military support (too many top flight weapons) they might be tempted to expand into Russia. Russia might just use their nuclear weapons (if they’re still operational) if their own country is invaded. No one wants that.
NATO’s intelligence resources (mainly UK, Germany and Poland I think) seem to be getting good information, and keeping the fighting restricted to Ukraine.
The joker in the pack is internal pressure within Russia though. I hope that the NATO intelligence is well informed there. Who’s to say that someone in Russia won’t use nuclear weapons on Russians in a civil war?