Has the US or NATO even threatened to go to war if Russia starts using tacnukes in Ukraine? I haven't seen any reports to suggest so, and I've been following the news on this war closely.

(The US has threatened more sanctions if Russia uses WMDs. That threat was in response to signs Russia was getting ready to use chemical weapons.)

My uneducated guess is that the primary goal of the latest rhetoric about the possible use of chemical weapons is intended to make this more clear. If "we" (not the US yet, but other countries that are nearer) are signalling a readyness to respond kinetically if chemicals are used, surely the message is that nuclear would trigger an even greater response.

China might just drop their support of Russia if that happened.

If a tactical nuke is used, then it is proof that escalation to nuclear weapons does not imply the global nuclear holocaust that has been acting as a deterrent.

It eliminates the logic of not going to war.

Not that "logic" has any connection to "appetite" or that excuses require it.