There is a libpurple/Pidgin plugin for Discord. As you might expect, however, it only exposes text chat and not any of the "rich" features such as voice chat or emote reactions.

I don't know of any free software full reimplementation of the Discord client, and I suspect such would be against their terms of service.

No, but there are client + server alternatives that are free. Mumble / Murmur is one. Rock solid and scales to large numbers of users even when running on a tiny VM. They are kindof slow to release new features, so you would want to try their latest snapshot of the server and client.

I haven't used Discord in a while, but when I did I just ended up logging in via a browser. (The binary blob client had some major cpu/memory issues when I tried it.)

It's essentially a browser binary.