>”English” may not be prescriptive (odd idea, that)

Natural languages can’t be descriptive or prescriptive, they merely are. Grammars for those languages can be either descriptive (i.e., describing the language as used by speakers) or prescriptive (i.e., prescribe how the language should be used).

In the former case, it’s perfectly fine to freely split infinitives; in the latter it is something up with which one does not put.