Currently I regularly do XML backups for all wikis that are deleted and store them in my homedir for a period of time (usually around a year and a half). These backups are sometimes requested by users who didn't realise their wikis were deleted and are relieved that their content wasn't lost.
Rather than having this informal process I'd propose we do the following:
- Generate XML backups for all wikis as part of the deleteWikis.php process right before wikis are permanently deleted
- Store these XML backups on mwtask* for around 18 months
- Automatically send emails on permanent deletion to all bureaucrats informing them that backups are available for 18 months and how to recover them.