Keep rules in a local file

Is your feature request related to a problem? Please describe.
In #mjolnir:matrix.org, I was told the list was entirely based on a room. This leads to an issue - on servers where the admins don't want their storage to be completely filled up after a few months, there will be a max lifetime for events. Any rules older than that will just be forgotten once they drop out of RAM, that is, after a sync command or after a restart (and yes, the latter will be necessary if you don't want to miss out on kernel security updates and such).

Describe the solution you'd like
Just keep a file for each list somewhere in /data/ that contains all the rules. Could be JSON, YAML, TOML, SQL, plaintext…

Describe alternatives you've considered
The alternative is that spammers and trolls (as well as their sockpuppets with similar names, if wildcard bans are used) will be able to join other protected rooms where they hadn't joined back when the rule was still active.

Additional context
https://matrix.to/#/!WpbOWAblxueZXAAnjj:matrix.org/$ovbL-Aa7o8x1GdTyBsa0FS4vbc02GVdU0TQwswmJSe0?via=matrix.org&via=privacytools.io&via=vector.modular.im

Start by tracing how moderation lists are currently loaded from Matrix rooms and where list state is handled. Define the persistent format and location under /data; done means rules remain available after sync commands, restarts, and room event expiry.