Maintained fork of powercord - a lightweight @discord client mod focused on simplicity and performance.
See the installation page of the Replugged Wiki.
Long story short... yes. Replugged is against the Discord Terms of Service — but, you should keep reading:
As of right now, Discord is not going out of their way to detect client mods or ban client mod users. On top of that, Replugged does not make any manual HTTP requests unlike certain client mods / plugins, so your client's user agent is the same as a legitimate client. Meaning, Discord doesn't detect a client mod like Replugged. They can go out of their way to start detecting it, but they don't.
Hypothetically speaking - even if they somehow did detect Replugged, users are very unlikely to be banned on sight. It doesn't make sense for Discord to start banning a substantial part of it's userbase (client mod users) without any kind of warning. Not to mention it is mandatory for Replugged plugins to be fully API-compliant and ethical, implying Replugged users can't be banned for indirect ToS violations (e.g. selfbotting).