This is all quite interesting. Of course language evolves.
But this puts an interesting slant on it
It's normal to say an heir. I can't imagine anyone saying * a heir*
Same with (an/a) historic event. An sounds much better than a
Found this which explains
*An is the form of the indefinite article that is used before a spoken vowel sound: it doesn’t matter how the written word in question is actually spelled. So, we say ‘an honour’, ‘an hour’, or ‘an heir’, for example, because the initial letter ‘h’ in all three words is not actually pronounced. By contrast we say ‘a hair’ or ‘a horse’ because, in these cases, the ‘h’ is pronounced.
Let’s go back to those three words that tend to cause problems: historic, horrific, and hotel. If hotel was pronounced without its initial letter ‘h’ (i.e. as if it were spelled ‘otel’), then it would be correct to use an in front of it. The same is true of historic and horrific. If horrific was pronounced ‘orrific’ and historic was pronounced ‘istoric’ then it would be appropriate to refer to ‘an istoric occasion’ or ‘an orrific accident’. In the 18th and 19th centuries, people often did pronounce these words in this way.*
But it doesn't matter