demilitarized zone
That proposal, envisions a 30-40KM buffer zone along the current front line. Not turning the entirety of the country into a DMZ (which is actually what we want, but are unlikely to get).
A line 30-40KM wide, would mean we wouldn't be able to shell each other with artillery very effectively.
These 30-40KM would be occupied by EU soldiers, whose job would be to ensure that neither side violates the ceasefire.
After the hostilities in 2014, we had European observers sitting between us and the Ukrainians. They failed miserably at doing their job, because Ukraine continued shelling Russian controlled territories throughout the entire period of time leading up to the 2022 invasion.
On several occasions, we even caught the "observers" using their UAVs to scout our lines on behalf of the Ukrainians.
Personally, I think both sides would ignore whatever Europeans are sitting in the middle, in the same way that Israel right now ignores all the UN peacekeepers in southern Lebanon and just continues with its job. All those Austrians, Irish, and others are sitting scared inside their fortified bases - absolutely powerless to stop the Israelis.