Room history retention settings / state event `m.room.retention`

Read that, it explains what is actually done etc.

Notes

Not part of spec. https://github.com/matrix-org/matrix-spec-proposals/pull/1763
Matrix uses milliseconds, keep that in mind!
Everything requires every server involved to have rention support enabled, otherwise the messages go nowhere and are fetchable upon request.
86400000 = 1 day (seen in min lifetimes)
Helpful: https://unitchefs.com/milliseconds/days/ / https://unitchefs.com/days/milliseconds/