I would bet that there’s a rule that not only says what you said, but redirects people to something like r/askplumbers or whatever for these kind of posts.

I haven’t used reddit regularly since the API exodus, but I was part of plenty of communities like that. Mods can’t allow exceptions because you’ll get regulars complaining about the rule breaking content and new users complaining that their post was removed.

Like you said, they were mostly professional subreddits, but others had similar rules (like r/churning, but they were extra crazy. They’d require all discussion to be in specific threads so the content was less likely to be indexed by search engines).