Well, for example the use of they as singular dates back as far as the use of they/them existed in English, shortly after it was borrowed from Norse languages spoken by Viking invaders. Written references are abundant, right up through Middle English and Early Modern English texts by none other than Chaucer and Shakespeare. Using they as a singular pronoun in native English today feels perfectly natural to most native speakers, who use it all the time.

Splitting infinitives and ending sentences in prepositions also feels perfectly natural -- because it is. It's native, ordinary, organic English as English has pretty much always been spoken.

But starting in the late 18th and continuing through the 19th century, grammarians - i.e. educated people grounded in Latin and Greek, which all educated people studied in those days - began to codify rules of grammar for English based on the grammars of the classical languages they knew but ordinary people did not.

Grasp of the arcane rules they invented served as gatekeeping, separating the uneducated masses from the upper classes who could afford long years of schooling for their children.

The rules they invented did not, however, serve English very well. This is one of the reasons why many of us tend to very naturally say things that FEEL right but are technically not "grammatical."

When we speak of "prescriptivist" grammar, we mean a grammar system by which rules are prescribed - handed down from on high - rather than rules that describe how the language actually works as in descriptivist grammar.

Sometimes prescriptive grammar works better in some languages than others, as in French where formal grammar rules are fairly well suited to the language as most people actually speak it.

But in English, formal prescriptivist grammar tends to be very ill suited indeed, one of the reasons many struggle with it and why it can still be an effective gatekeeper.