I don't know the answer, but the note reads:
- Note that Meta-Wiki does not back up deleted revisions, and they may be lost in the event of a serious server failure.
If Meta uses the same user interface options as every wiki else, then deleted revisions do stay in the database. If they did not, they would no longr be restorable and the same note also states that admins do have the possibility to restore deleted revisions.
What I could think of is this: When a revision gets deleted, its revision information is internally moved to the database table "archive". Revisions from that table will still be restorable just as usual. If however, during the creation of backups this table is excluded, then the information would be lost, if a backup was used.
If the backup script still excludes the archive table today, is what I don't know. Aren't the database dumps available somewhere? Looking into them should help clear that up...