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

Read that, it explains what is actually done etc.

Notes

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/