I really doubt that NATO would be willing to enforce this no-fly zone in a non-NATO nation, that would mean being willing to shoot down Russian military aircraft, which would surely escalate the war.
Simple fact is western powers don't have the capability to maintain any sort of airspace over ukraine or other nearby countries.
Russia is not a weak country that can be deterred by a no fly zone.
Read some latest defence blogs, Russia not only has the best air defence systems but leading electronic warfare technologies.
Many countries buy Russian weapons even facing western sanctions. They are cheap and effective.
What you're describing is setting up a shooting conflict between nuclear powers. That is bad. Like really bad. It doesn't matter who shoots first because everyone dies.
That cuts both ways. NATO air assets over Western Ukraine would have forced Russia to start a shooting conflict with a nuclear alliance.
As is, they were able to move into Ukraine without nuclear risk, because the US explicitly said we wouldn't put troops there.
And let's not pretend Russia shooting down a US plane or vice versus would start a nuclear war. It happened fairly commonly during the Cold War, quietly.
NATO forces in or above Ukraine would be the modern equivalent of the Cuban Missile Crisis, and we would be the aggressors setting up shop right next to a nuclear power. A lot of our fighter aircraft are nuclear-capable, and the Russians would have no way of telling which missiles are conventional-tipped or nuclear-tipped.
What you're describing is reckless brinksmanship. One mistake or misread and the world ends.
NATO forces have been above Ukraine for the last several years. The US et al. were flying ISR missions right up against the borders of Belarus, Russia, and eastern Ukraine rebel-held areas up to a couple days ago.
"Reckless brinkmanship" is an odd phrase, when we're talking about responses to Russia invading a sovereign country.
But NATO doesn't really want to fight that war. Electorate would vote governments out for getting involved in war they doesn't really care about. And Putin knows that too.
So what if NATO puts some forces in Ukraine, but Putin calls their bluff and attacks anyway? Small contingent wouldn't be nearly enough to hold the territory, so you need to either engage for real (but you don't have political capital for that), or withdraw forces which would look like huge humiliation.
And let's not pretend Russia shooting down a US plane or vice versus would start a nuclear war. It happened fairly commonly during the Cold War, quietly.
This is not the cold war, I don't think anyone can predict what Putin will do if provoked and he needs to save face.
You are forgetting the fact that the odds are heavily against NATO, in a RAND wargame it was found that russia can occupy the baltic States within 48 hours while nato would be still warning up
NATO should have been requested to by Ukraine and declared a no fly zone for military aviation west of the Dnieper, before Russia invaded.
An inability to use aviation would have severely slowed Russian plans.