One of the things about ZFS that bites people every so often is that you can't remove devices from ZFS pools. If you do 'zpool add POOL DEV', congratulations, that device or an equivalent replacement is there forever. [...] At this point you might well ask why ZFS can't do this, especially since many other volume managers do support various forms of shrinking.
The simple version of why not is ZFS's strong focus on 'write once' immutability and being a copy on write filesystem. Once it writes filesystem information to disk, ZFS never changes it; if you change data at the user level (by rewriting a file or deleting it or updating a database or whatever), ZFS writes a new copy of the data to a different place on disk and updates everything that needs to point to it. That disk blocks are not modified once written creates a whole lot of safety in ZFS and is a core invariant in the whole system.
