Explain your idea

I think a good idea for the plugin to have would be on Startup to do a check to see if whitelisted players are still in the Discord. If they are not they should be removed from the whitelist. This of course could be turned on and off in the config file. If you do implement this I really hope you will do it for 1.12 as well

If this is platform-dependant, which platform is this about?

Forge, Spigot