It's only that until 19.07 there was no STABLE branch, only RELEASE and when I tried to update from 19.07 to 12 U2 ...
I think the idea is to never update in the current BE,
always update into a new one, so you can roll back to where you were if things go wrong.
Older BEs can be removed when you're sure everything's working OK.
BEs don't take as much space as one might think, because they are ZFS snapshots;
the space used is only the diff between the old and the new.
As JG hinted, you can also ZFS-snapshot whatever you want whenever you want,
as a safeguard against accidental deletions and bad changes.
A new snapshot takes no space at all; the space used grows when you alter files.