"purple-discord" is the only free software Discord client I know about. It is a plugin for Pidgin and other libpurple based messengers such as Bitlbee.
"rdircd" the "reliable Discord client IRC daemon" might be an alternative but I've never used it.
Neither of these run out of the box on mobile but you can run them on a server you control, or maybe you could run them under Termux (have never tried this).
You can also use a Matrix or IRC bridge if you control the Discord "server."
I don't count client mods or web wrappers as they are fundamentally coupled to the proprietary client.