Your comment proves you don't understand what's happening. At all.
Trump defamed someone. That's a civil issue. He was put on trial. On a civil trial. Because it's a civil issue. He lost. He got fined. Not sent to jail. Because it's a civil trial.
You'd still have to handle transferring keys across your devices, every time you login on a new one.
Also, searching would probably not work, at least as well as it does right now, since all messages are indexed on the discord side, which they wouldn't be able to do without seeing them. Everything would have to happen on device, meaning the devide would have to store all messages.
Have you tried using Signal on desktop? It doesn't offer history syncing. Cross device for whatsapp for example is also a terrible experience. Unusable for something like Discord.
For a seamless experience Discord would probably have to store the keys themselves, defeating the whole point.
You're actually joking with the "inviter providing the decryption keys to the invitee" part right?
The whole point why people use discord is that it's simple, this is a feature that'd only annoy the average person, and every single extra step is a disaster for user retention (look at any eshop study).
Stuff like this is completely irelevant to discord, the tiny subset of people who actually care will and should use Matrix / other solutions, because that's the people they were made for.
Your comment proves you don't understand what's happening. At all.
Trump defamed someone. That's a civil issue. He was put on trial. On a civil trial. Because it's a civil issue. He lost. He got fined. Not sent to jail. Because it's a civil trial.