I had a fight against the AI where every support non of which historic caused Mishra to replicate. Things like Gilded Lotus, Pirate’s Cutlass, Dagger of the Worthy, etc.
Those are all artifacts and artifacts are historic cards; thus, Mishra is working as expected 
Seriously? Then why do they put legendary icons on some supports but not all of them? Oh well if it’s working as intended then disregard I guess. At least the stupid Waterveil card requires a landfall, although not hard to achieve either. Everyone wants that instant win!
Haha yeeeeeah.
So historic is an umbrella term for legendaries, artifacts, and sagas. However, legendaries, artifacts, and sagas are not equal.
Legendaries are usually special named locations, objects, or entities. For instance, Etali is a specific dinosaur with a name like a hero; therefore he is a legend. You can consider them famous people/animals, etc. Kefnet’s monument is another example - its not just any monument its Kefnets (which is also a legendary creature). Granted not all legendaries hold a special name (pyromancers goggles).
Artifacts are special objects like objects of ancient civilizations in real life.
However some artifacts are legendaries while others are not. Immortal Sun is both an artifact but its also a legendary. This is probably because there is only one in existence as opposed to meteor golem, an artifact but not legendary. I would guess there are numerous meteor golems (and no hero version) unlike Immortal Sun.
Though… Pyromancer’s Goggles, while not being specifically named, are a specific pair of goggles. They’re the goggles worn by Jaya Ballard, hence why they’re legendary.
But yeah, you covered it well with the initial explanation that all artifacts are considered as historic.
Oh! I did not know this. I was going to say they were Chandras but felt it was too speculative so deleted it. But thanks for that tidbit!
Well.. Chandra stole them once, but returned them to Jaya
Just to clarify for you, that icon is for Legendary, not Historic. All Legendary cards are Historic, but not all Historic cards are Legendary.
Also, if you think it’s out of control now, you should be glad they didn’t directly copy it from paper.
That would have it effectively double itself for each trigger.