This is claimed all over the internet, and I am highly, highly, highly skeptical about this reasoning...just seems like a bunch of wokes wanting a reason to criticize old timey white european traditionalist academics without actually putting their research in.
I tried reading through Robert Lowth's grammar book, and although he spoke a lot of the infinitive, I couldn't find the section about split infinitives.
I'm just curious if any 18th--or whatever--century grammarian actually said "You are not allowed to split the infinitive in English because I really, really like Latin a lot". I'm sure that they did prescribe against it, but I doubt that reasoning.
I have seen other sources say that there's no evidence that the prescription against the split infinitive is based off foreign languages.
Many people who ought to know better—"authorities" on English—declare that the objection to separating to from the infinitive verb that follows is based on Latin (or some other language) where infinitives are single words. If some purist has made such a comparison, I can find no record of it. Henry Alford (The Queen's English [1866]) thinks the to and the verb are "inseparable" but he does not mention foreign languages (p. 227). This idea is part of the folklore of linguistics.
The reason I'm so skeptical about this is that it seems to reduce actually rather brilliant academics from the past to latin-obsessed simpletons who didn't seem to realize that not only didn't the romans split the infinitive...they physically couldn't!
Thanks for your assistance.
The belief that it comes from Latin is most likely from association with the fact that a number of supposed rules and interpolations for English came from a belief in Latin's supposed superiority as a language; the "rule" resembles that other prescriptions of the type, so it became tarred with the same brush.
However, as you say, there is no such thing as a split infinitive in Latin, so that origin is dubious. The early origins of the prescription seem to be based on the idea that split infinitives somehow denote lower class or to be entirely arbitrary. For example, John Comly’s English Grammar Made Easy to the Teacher and Pupil (1803) specifies that "[a]n adverb should not be placed between a verb of the infinitive mood and the preposition to which governs it". This later became subject to the rather hysterical moods which governed almost everything in the 19th century.