I didn't know about "an historic" until I heard it from American journalists. They did pronounce the 'h' and it did indeed sound very wrong. I believe I first heard it from Rachel Maddow. I can't find any clips, but there are a bunch of transcripts from her show if you search up:

> Rachel Maddow "an historic day"

For example:

- https://www.nbcnews.com/id/wbna47356492

- https://www.ms.now/transcript/2013-11-21-msna738766

It's one of those things American pseudo intellectuals do because they think it's more correct and want to sound superior. Maybe they heard a professor say it that way or read it in a book. Anything but actually process why the a/an distinction exists.