[Based on my comment.]
We also have best-practices and rule-of-thumb (and even expertise, which was used only once!). Since the purpose of this site is to provide guidance, expert opinion and thoughful recommendations, I don't see any interest in keeping the recommendations tag alive.
As @Nick pointed out, there is no connexion between 'recommendations' and 'recommender system'. I would suggest killing the recommendations tag, and cleaning up the aforementioned threads.
Edit (6 Sep.)
I've removed the tag from the remaining threads. It can safely be blacklisted now.