As far as I know, __newindex is not fired when you remove a value from a table by setting it to nil, you may be better off doing this:
function mt:remove_player(player: Player)
if not player then return end --> Checking if player exists.
self.players[player] = false
end
This should still work with your if exists logic, as if statements like that will go off if a value is equal to something that’s not nil or false.