Just curious -- where in the TOS are third party clients forbidden? I've read it a few times, and I'm not seeing it.

Probably:

> You agree not to (and not to attempt to) (i) use the Service for any use or purpose other than as expressly permitted by these Terms;(ii) copy, adapt, modify, prepare derivative works based upon, distribute, license, sell, transfer, publicly display, publicly perform, transmit, stream, broadcast, attempt to discover any source code, reverse engineer, decompile, disassemble, or otherwise exploit the Service or any portion of the Service, except as expressly permitted in these Terms;

It's a catch-all but the best restriction that prevents third-party clients is probably 'prepare derivative works based on' and 'reverse engineer' (which you would need if you want your third party client to use any regular client API calls, or if you want to support signing in with the user-facing login page/qr login).

A third party client like Ripcord isn't a derivative work.

It also wasn't made by reverse engineering any of Discord's code. Even if it were, these rules still wouldn't apply to using third party clients.