It seems that Second Thought (and their second channel, First Thought) has been silently removed from Nebula, with no comment from them or anyone on Nebula’s side.

There’s some context in the comments of a couple of reddit threads about this, that have been deleted on the Nebula subreddit:

https://www.reddit.com/r/Nebula/comments/17be8bf/second_thoughts_channel_being_removed_from_nebula/

https://www.reddit.com/r/Nebula/comments/17bav9j/what_happened_to_firstsecond_thought/