When you do writing as a form of art, rules are meant to be bent or broken; it's useful to have the ability to explicitly write new ones and make new forms of the language legal, rather than wrestle with hallucinating LLMs.

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.