we will never be able to have all the skins???
Some skins have returned, but others have not.
Eg: Khadgar was first available for a short while in 2016, and then for the next 4 years only available through limited special promotions in Asia (you had to buy tea bags or something)
He was finally made available again in the Hearthstone shop from 26 Feb 2020 to 4 Mar 2020.
This video pretty much has all of the limited ones (hero skins start at 2:43):
Then there are the 1000 win portraits, which you can earn by playing… a lot.
And the Tavern Pass Level 50 skins, which you can choose one of.