The deterrence could weaken but I suspect a single use of a nuke could (assuming it doesn't MAD) bring back deterrence by normalising use somewhat.

If you think about post hiroshima...I don't think anyone doubted the US was willing to wipe out cities

So not entirely convinced it's dead

It’s conceivable that Russia could force the West to back out of the proxy war. But then they’d get proliferation in Europe, Taiwan, South Korea, and other places. And Usrael would very likely use nukes against Iranian missile cities. (Conversely, first use against Iran would give Russia license etc.) Apart from general global instability and uncertainty this might unleash.

So, it’s not hard to see why they haven’t—so far. And why they are pursuing their (slow and costly, sure) conventional military, economical, and demographic long-term neutering of Ukraine. And let’s not forget the enormous economic/budgetary cost to the EU.

Also, Russia retains a number of ways to escalate inside of Ukraine: bridges, dams, depopulation of frontline cities. China could finally abandon their stance of being mostly neutral.

The problem is that Ukraine/West keep escalating and the day when Russia’s calculus changes may yet come.