Apps need to add support for the new portal system. Chromium is adding (or added?) support, so Discord may implement it once they use an Electron version with support.
If you’re using KDE, you can tell Discord to use X11 and use KDE’s feature to let X11 apps snoop on key presses.
Are you sure about that? KDE has a feature that lets Xwayland apps snoop if certain keys are pressed, but Gnome does not.