I found a remark in an article here saying " any old regions that are marked for garbage collection will be removed and reused for new Region s afterwards". So it seems that regions can indeed be deleted in this way by dereferencing them.
It’s not clear though how usable that is. Say I have a large region which dominated my canister’s memory footprint, dereference it, then create a new large region. If the garbage collection does not run in between the two events then I accidentally double my canister’s memory footprint and will pay storage cost for the doubled memory forever.