Looking at the revision of a closed proposal I can see that the last version before the close message (Which is the only revision.) has the rollback button enabled.
Hovering your mouse over it will give a pop up saying "sets the current revision to this version of the post, resetting any offensive votes", What here can be reverted and what offensive votes?
A proposal that is not closed does not have this link enabled:
P.S: This is not limited to just one proposal only and also shows in closed example questions with only one revision.