SquaremapShapes is a mod that allows you to add markers to your squaremap instance, and are persistent between server restarts.
This mod is designed for usage on a dedicated server, however it will still function on singleplayer.
Squaremap is a required dependancy.
Common issues:
If you accidently set a marker with the same ID twice, it will likely crash your server on the next world startup. This is primarily a singpleplayer issue, but not exclusive to singleplayer.
To fix this, either delete one of the two markers via commands, or manually nagivate to the SMSMarkers.txt located at <ServerDIR>/squaremap/SMSMarkers/ and remove the duplicate entries.