It's nothing specific to bots. Discord's TOS doesn't allow you to use a thirdparty client. This has been the case since day 1 (I think) but they haven't been enforcing it. Now the creator of this client has had their account banned, so it seems that Discord may be cracking down on the use of unofficial clients.
Mind you, Discord's ToS do not specifically mention third party clients at any point. They do say that they can ban your account for whatever reason, however.