Otherwise, I've never seen Boeing "forbid" or "prohibit" anything.
Indeed. I've flown only Boeing airliners, of various types and for various operators in different countries. It was interesting to see the differences in some procedures and use of this or that. When I asked how XYZ was allowed here, and not in my previous airline, the answer was, "we asked Boeing if they had any objection to this or that and they said no." So a 'not recommended' or some such became an OK if you asked properly.
If anyone can accept a little drift I have a few other technical questions regarding other 'not recommended' and wonder about what operators do.