It was more than just the phone books. Back before smart phones, if you needed to look up a phone number you’d call information (411) and they’d look it up for you. For instance, if you were stuck on the side of the road and needed a tow truck.
Information would be able to look up businesses close to where you were using the NPA/NXX of the phone number you were calling from (the first six digits of the number including the area code) and then give you a couple options in alphabetical order.
I had a client who had a phone number in every exchange in NYC and had a name like “AAA Towing” so no matter where in NYC you called information for a tow truck from, they’d usually be the first option given to you.
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).