Is there a rule about linking to other message boards?

IANAMod, but it’s been my understanding since shortly after I joined that context is everything. If anything is pejorative of another board as a board, it’s against the standard usage here. This goes squared and cubed in spades for anything that might instigate a board war. However, a mere reference to discussion elsewhere that does not put down that board, even if it might be negative against a given member of that board, is not considered wrong. I’ve linked to discussions on a number of boards that helped to clarify or amplify a discussion here, and even pitted a number of people who were taking a stance on another board which I differed with strongly, making clear that I was not complaining about the board itself but some of its members.

FWIW, if “the flatbread house” is what I think you mean by it, it had a longstanding good relationship with this board, with staff on either board members at the other. It was itself founded by people objecting to poor moderation of the I-believe-now-defunct “sinister buttock” board, with which this board had a New Years Eve encounter years ago, and seeking to provide an alternative.