First, you may want to get a backup of all your messages and servers, using Discord Chat Exporter (there is a package for it in the AUR). The documentation for usage on GNU/Linux is not so complete, I figured (on Arch Linux), I had to cd in /opt/discord-chat-exporter and then run dotnet DiscordChatExporter.Cli.dll <commands> (you must have dotnet installed, and have the correct version of it)
You might also want to ask for all the data that discord collected on you with this guide discord support - requesting a copy of your data. Warning: there is a 3 month delay between asking for the data and receiving it, for some unknown reason. It's probably just to be annoying so that less people do so I guess.
I also advise you to delete all your messages using undiscord, since Discord DOESN'T DELETE ALL YOUR MESSAGES UPON ACCOUNT DELETION, instead the message author is replaced with "Deleted User" (which is laughable, with enough messages it is trivial to guess who sent what). Careful, using undiscord breaches Discord's Terms of Service, so just to make sure set the delay between deleting messages to something reasonable (discord could delete your account for bot automation, thereby stopping you from deleting your messages). The default settings should be fine.
Then, send a message to your contacts stating why you are deleting discord (you can just send them a link to this page) and where they can find you afterwards. This is an important step because it contributes to inverting the network effect, which is the number 1 reason people continue using proprietary disservices.
TODO
If this guide was helpful to you, send me email, or for members of tilde.town, sign my guestbook!