There are no plans to re-release any past promotional skin at this time. Demon Hunter Sombra was the only exception because a web blog post on the official BlizzCon website specifically said it would be re-released in 2019 (which that promise was fulfilled). (Click here for details.) Mini-Event skins, OWL Legendary Skins, Pink Mercy, etc. are all expected to not have any plans to return.
Could they come back? Sure, but please do not hold your breath… (I don’t know how to do CPR.)