For what it's worth, I don't think there is any known case of a Discord user being banned for using an unofficial client yet.

https://vencord.dev/faq/#Will-I-get-banned-for-using-Vencord...?

There have been cases and a whole project was shut down because of it https://github.com/Bios-Marcel/cordless?tab=readme-ov-file#i...

However this is an entirely different question. Whether or not an API is copyrightable, and whether you'd be in violation for recreating a proprietary API for the purpose of creating an (open-source) market alternative for their product, are entirely different questions to whether or not a specific private company has stopped serving specific customers due to use of software they do not authorize (which they are entirely in the right to do, legally)