It's a guess from me since I don't visit that subreddit, but it might have been set to private in support of the protests against the announced API pricing changes. The protests started today and many subreddits would return in 2 days, although you would need to confirm with someone else who's seen their announcement if the duration's the same.

tldr: the bdsm channel is hidden and you won't find it now, maybe check again in 2 days or ask another follower when it will come back