Not an insurmountable problem, ChatGPT will use "aight fam" only in context-sensitive ways and will remove it if you ask to rephrase to sound more like a professor, but RHLFing slang into predictable use is likely a bigger potential challenge than simply ensuring the word list of an open source program is sufficiently up to date to include slang whose etymology dates back to the noughties or nineties, if phrasing things in that particular vernacular is even a target for your grammar linting tool...
aight, trippin, fr (at least the spoken version), and fam were all very common in the 1990s (which was the last decade I was able to speak like that without getting jeered at by peers).
Also, it takes at most few developers to write those rules into a grammar checking system, compared to millions and more that need to learn a given piece of "evolved" language as it becomes impossible to avoid learning it. It's not only fast enough to do this manually, it also takes much less work-intensive and more scalable.
Or, in other words: if you "just" want a utility that can learn speech on the fly, you don't need a rigid grammar checker, just a good enough approximator. If you want to check if a document contains errors, you need to define what an error is, and then if you want to define it in a strict manner, at that point you need a rule engine of some sort instead of something probabilistic.
(^^ alien language that was developed in less than a decade)