> most other grammar rules are broken pretty often in really informal settings

I don't think this is true. There are lots of grammar rules that are followed in any setting. They have to be because they're a big part of how we make ourselves understood. Generally if people don't want to be understood they don't bother speaking at all.

The grammar rules people like to talk about for fun are the ones that are frequently broken, and may even be fake rules that were just made up to give grammarians something to write about.

Real rules that everyone follows are taught to language learners but are not very entertaining for people who already speak the language - like the rule that singular improper nouns must have an article or other determiner, but determiners are optional for plurals and proper nouns. Or the rule that verbs must be inflected for past tense.

I think adjective order is distinctive in being a rule that feels surprising even to native speakers - we imagine we have a free choice and then enjoy learning that we've been following a complex seeming rule that we didn't even know existed.