Systemli's Matrix home server instance has a default retention time of 30 days. We allow values from 1-180 days.
Clients that have already downloaded and stored the event/message might continue to show it, unless they themselves implement support for message retention policies, the homeserver can not do anything about this.
It is possible to send custom events from developer tools of element web and set room retention time there.
The developer tools can be opened by typing /devtools in the message editor. (With older versions of Element, it was something like: Right-click a room → Settings → Advanced → Open developer tools → Send custom event)
In the devtools, select “Send custom timeline event”
You'll first need to convert the desired time in days to milliseconds, i.e.: 1000*60*60*24*<number_of_days>
Event Type:
m.room.retention
Event Content:
{
"max_lifetime": 2592000000
}
Click “Send”
After sending, you can verify that the retention is set: Go back, choose “Explore room state” and m.room.retention there.
Source: https://github.com/vector-im/element-web/issues/3800#issuecomment-1030568445