The mod

This modification adds one "command" (/pvp) when ran it will toggle the player's ability to pvp, by default pvp is disabled.

It saves to disk at ./config/pvptoggle/pvptogglestates.json and saves space by only saving the players who have pvp enabled.

It saves the UUIDs of players so the persistence feature will not work in offline mode (unless under a online mode proxy) and I don't support offline mode other than when using proxies.
If you want to you CAN use offline mode but beware I will not provide any support for offline mode servers.

I would say this mod is fine for up to 1000 players (depends on how many have PvP enabled).
Just because this modification is in Alpha doesn't mean it isn't usable!

Commands

/pvp - Toggles PVP