AN historical is grammatically incorrect.
Historical begins with a consonant. You use “an” when the following noun begins with a vowel eg A house, an element.
An historical is an affectatious enunciation, someone trying to sound more sophisticated than they are.
Grammar nazi mode off.
Noticed that when I lived in the UK. Seems to have trickled over into our lexicon as of late. As well as speaking in an affected language in particular pronunciations. “Pok-eee-srahn”. Absolutely cracks me up.
I guess I'm older than you are. I was taught that either was appropriate. A little googling showed me that in the US, up until about the late 1940s, actual usage was 50/50. Then it started to trend to mainly using "A" before words beginning with "h." It is speculated that there might be a regional difference. It some parts of the country there is a tendency to drop the "h" in words of 3 or more syllables, making the use of "AN" more natural.