I waited a while after creating this mod meaning to come back to it with a different perspective.
Three things stand out:
1 - There seems to be a bug in setting the reserved_slot parameter in a server's conf file.
2 - Do we really need yet another priv to be added to the long list of privs a server admin already has? Maybe just let someone with another priv (which one?) use /grant reserved_slot playername.
3 - This mod needs testing. I have had reports that it doesn't always work. I think the server is not synchronous when it accepts or rejects player logging in.
Comments, critiques, and suggestions are welcomed.
Based on : https://forum.minetest.net/viewtopic.php?f=5&t=10037