Well, it's not the old one, but it is the Tablet variation of it - which is only possible because Discord developers did not make a tablet variation of the new UI. And since there are no flags pointing to that, I doubt they will introduce it any soon.
Anyway.
You will need:
- Magisk (you probably know where to get that, KernelSU should work as well if you have the Zygisk KSU module);
- LSPosed (Riru variant or Zygisk variant - only one at a time, and you will obviously need either Riru or Zygisk);
- App Settings Reborn and, obviously,
- the discord app.
Install LSPosed, reboot. Install the App Settings module, enable it in LSPosed, reboot. Install Discord if you have removed it while raging, go into the App Settings' whitelist in LSPosed and check Discord.
Now, open App Settings, open Discord's setting page and enable it. Set XLarge Resources to true and Screen Size (DP) to the maximum available value. Press the save button, agree, then go into Android's app info page for Discord and force stop it. Done.
There are a few things that are wrong with this. You don't get the old dock back. The buttons that were on the old dock stay aligned to the bottom of the screen in your user card tho, except Friends - its now in the DMs section, just like on desktop, and Server List, which is now the default view anyway. Also, you will need to force stop the app any time you want to change its screen orientation, since applying this mod makes it so Discord stops receiving display size update events.
Anyway, welcome to the club of I Use Unsupported BS Because The New Stuff Sucks!
@gravitos vendetta can do this with the experiments plugin without root too! :3