Threads in Slack are a great way to keep a conversation in a channel, yet not flood the main stream of conversation. But once you jump into a thread, you are immediately subscribed to notifications for replies to that thread.
Fortunately, you can unsubscribe from Slack threads. I wish Slack made this more obvious, but until then, all you need to do is type UNSUBSCRIBE in the thread. No forward slash command needed; in fact, I can’t get /unsubscribe to appear as a command.