If Russia would use nuke, they would have used it already. It's like 3 weeks since they declare the annexed part Russian. Ukrainian bomb it, hell, even Russia attacking Zaporizhzhia (Which they claim is Russian) and there are roll back after roll back and the Ukrainian even down half a Bridge in Crimea and set Belgorod on fire and into darkness.......
There is already more than enough "excuse" to have Russian send some Ukrainian way. Yet they still didn't, I mean what kind of red line we are talking about? Attack on Annexed Territories, Check, Taking Annexed Territories back, check, attack Critical Infrastructure in Russia, check, attacking Russian soil itself, check. I mean the only thing the Ukrainian didn't do is to roll up from Kharkiv and invade Belgorod themselves...
If there is a red line, that line had long passed.