Ok I already made the next update. The cvar changed into "rememberthefrags_resetseconds".
Set it to 0 and stats will be remembered forever.
Set it to 600 and users' stats will be reset upon new map, or if the map is still running, after 600 seconds from the time they left your server.
Set it to -1 and users' stats will be reset upon new map without being reset as long as original map is running.