when is it artistically (or otherwise) defensible to break grammar

[🐒Gods of Prosperity🔱🐲💸]

Not all grammatical rules are created equal. For instance, to not split infinitives is hardly necessary.  often neither are rules of capitalization  - which is properly orthography and not grammar anyway.  PEOPLE IN ANCIENT TIMES GOT ALONG WELL ENOUGH IN MANY LITERATE CULTURES FOR CENTURIES WITHOUT PUNCTUATION OR LOWER CASE LETTERS

when you run into trouble is when you cast aside rules in a way that leads to ambiguity - dangling clauses and such. unless ambiguity is what you're going for.  (that wasn't a sentence - does it bother you? I have to admit it does me sometimes)

but to some extent grammar is inescapable in the use of language

poetry often tries to be evocative by leaving subjects without verbs, or transitive verbs without objects. sometimes it works. often it fails, and the reader finds it inaccessible and void of narrative.