According to Monica Cellio's Meta SE answer to Is chat room transcript link permanent? (from November 2017):
rooms with little activity are auto-deleted, meaning that only users with high rep (10k on that chat server) can see them
But I can't figure out what the process is for seeing the content of deleted chat rooms.
I clicked on the comment containing the following link to a chat room:
Comments are not for extended discussion; this conversation has been moved to chat.
But I only got the "page not found" page, which I assume means the chat room has been deleted. I can't figure out how to view the deleted content, even though I gather I should be able to since my ELU account has more than 10k rep.
I wondered at first if I had to log in separately for chat (since there was a "log in" button at the top right) but I seem to already be logged in.