I find that clear-cut, rigid rules tend to be the least helpful ones in writing. Obviously this class of rule is also easy/easier to represent in software, so it also tends to be the source of false positives and frustration that lead me to disable such features altogether.
When writing for utility and communication, though, English grammar is simple and standard enough. Browsing Harper sources, https://github.com/Automattic/harper/blob/0c04291bfec25d0e93... seems to have a lot of the basics already nailed down. Natural language grammar can often be represented as "what is allowed to, should, or should not, appear where, when, and in which context" - IIUC, Harper seems to tackle the problem the same way.
Even these few posts follow innumerable “rules” which make it easier to (try) to communicate.
Perhaps what you’re angling against is where rules of language get set it stone and fossilized until the “Official” language is so diverged from the “vulgar tongue” that it’s incomprehensibly different.
Like church/legal Latin compared to Italian, perhaps. (Fun fact - the Vulgate translation of the Bible was INTO the vulgar tongue at the time: Latin).
Languages are used to successfully communicate. To achieve this, all parties involved in the communication must know the language well enough to send and receive messages. This obviously includes messages that transmit changes in the language, for instance, if you tried to explain to your parents the meaning of the current short-lived meme and fad nouns/adjectives like "skibidi ohio gyatt rizz".
It takes time for a language feature to become widespread and de-facto standardized among a population. This is because people need to asynchronously learn it, start using it themselves, and gain critical mass so that even people who do not like using that feature need to start respecting its presence. This inertia is the main source of slowness that I mention, and also and a requirement for any kind of grammar-checking software. From the point of such software, a language feature that (almost) nobody understands is not a language feature, but an error.
> You also seem to be making an assumption that natural languages (English, I'm assuming) can be well defined by a simple set of rigid patterns/rules?
Yes, that set of patterns is called a language grammar. Even dialects and slangs have grammars of their own, even if they're different, less popular, have less formal materials describing them, and/or aren't taught in schools.