Debian package lacks Desktop Entry file
Looks like the Debian package (to be clear, not the Snap) doesn't install a Desktop Entry file at all, so it doesn't show up in the menu.
This should do the trick:
[Desktop Entry]
Version=1.0
Type=Application
Name=RiseupVPN
Comment=Anonymous VPN
Comment[es]=VPN Anonima
Exec=/usr/bin/riseup-vpn %U
Terminal=false
Icon=/usr/share/riseup-vpn/icon.svg
Categories=Network;Application;
StartupNotify=true