Twitch has deprecated slash commands over IRC, as explained in the announcement post back in September https://staging-discuss.dev.twitch.com/t/deprecation-of-chat-commands-through-irc/40486
There was shutdown windows in the lead up to the full removal, but now slash commands are completely removed as announced. As such you will have to change your bot over to using the API endpoints instead, all of which is explained in the linked announcement thread.