Above, the arms of Cambridge University.
The University of Cambridge has an online dictionary (https://dictionary.cambridge.org/). Now as we all know, Cambridge University is a place stuffed full of very clever people, so it stands to reason that a dictionary compiled by these learned folk should be spot on!
Well, these learned persons define the adjective, “unacceptable” as; too bad to be accepted, approved of, or allowed to continue.
I agree with that definition!
I also think that the Ukrainians will deem the reported proposals a plan worked out between Moscow and Washington as unacceptable.
Herewith the BBC report entitled: US military officials in Kyiv as Europe warns against reported Russia peace plan……..
GOTO: https://www.bbc.co.uk/news/articles/cvgdgrqwnq9o
On further thought, I would add another adjective will suffice to describe what Russia has offered: The adjective, “grotesque”!