Option in config called Force-Voice-Chat or something.
When this turned on: users have to be in lobby channel when they want to join.
If a player quits discord or leaves a voice channel he also get kicked from the server.

--> you can only play when player is connected to voice-syctem of discordsvr

I don't think this was properly answered. Account linking is too simply to do preliminary setup. this user is asking that every time a user is in game he also has to be in the correct discord voice channel.. Forcing a player to use their voice... you could extend this idea to have some kind of voice captcha so they are encouraged to speak and not simply listen

I think a voice captcha would be too niche of a use case personally.
Also, Discord's API doesn't officially support receiving voice, so this could be hard to implement. JDA also has it as low priority as well: discord-jda/JDA#904

What about players who haven't been linked? How will this be implemented?

What about players who haven't been linked? How will this be implemented?

The framework for this already exists, this would be an addon to the require a linked account to play module so account linking wouldn't be an issue