*Shrugs* If you are talking pure communication a linguist will tell you the rules of grammar extend only as far as being understood. However; if you are talking about a technicality of writing as a form, either technical or artistic, the rules are laid down and very clear. 95% of people probably don't ever need to know the rules because they don't write or perform in that arena, but, if you call out someone that is working in that arena, you need to be correct in your assertion.
The vernacular can change rapidly, but the rules that govern grammar in writing change much, much slower. You are talking hundreds of years. It is why you can still read Moby-Dick and understand it.