Legendary no longer means "super powerful one of a kind pokemon of legend" any more.
While it can and does mean that (Solgaleo and Zacian being recent examples) I think they could easily be considered "Mythical" all the same. The Legendary brand at this point is just anything GF classifies as rare or with limited access in a players save.
I think UBs (mostly Poipole) Kubfu or even Type:Null perfect examples proving this, you get one per save/character but it's just a regular Pokemon with evolutions and mild almost starter tier stats despite being marked as "Legendary" in the game files. If Porygon was released today it would be receiving the same """legendary""" status that Silvally does.
It's a term that has lost most of its meaning after being thrown around willy nilly by TPC and fans alike for anything vaguely rare in the lore and kind of imposes a stigma on even the shittier members. Imagine throwing out something like Guzzlord just for your friend to say "ugh stop using legendaries that isn't fair".

>tl;dr "legendary" should have been reserved for boxart tier mons like Rayquaza and Dialga