Install it locally in your Home so you can do the automatic update.
Use the AUR.
Use Flatpak.
Use Snap.
Use the workaround to change the version in the config file.
Use your web browser.
Use Discord Canary (I guess).
Use unofficial clients.
Plenty of solution for your “emergency”.
You can’t blame Manjaro for not updating Discord as soon as Discord release an update and block users from using outdated/previous client. You can however blame Discord for locking users out of Discord when they push an update and forbid users to continue using their services if they do not download latest patch.