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Users should be able to have 'fstrim' run on a file system backed by thin-provisioning when the respective thin-pool crosses the fullness threshold. This can allow the thin-pool to free up discarded space - potentially dropping the pool below the fullness threshold again without having to grow the pool.
This feature has been provided in 7.4. The user can run anything they want if the produce a script and point lvm.conf:dmeventd/thin_command at it. Closing my own bug here.