TL;DR: Does the Celestial Armor require medium armor proficiency?
I found the Celestial Armor which said that it is a medium armor Chainmail+3, and is light enough to be considered as light armor.
But, does it work just like a Chainmail made from Celestial Mithral, which then makes the armor light enough to be considered as light armor, but still requires medium armor proficiency? After all, it does seem to be made from it, even if it doesn't state it.
When checking other armors, I found the Elven Chain which state that it is a Chainmail, but should be considered as light armor, even from the proficiency point of view. But it also state that it is made from Mithral unlike the Celestial Armor.
Best Answer
Paizo creative director James Jacobs in a 2010 Paizo message board post, in part, says
Thus, according to Jacobs, this FAQ entry on proficiency with mithral armors doesn't apply to celestial armor, making it so folks who are only proficient with light armor can wear the celestial armor normally.
While not a rules guy, Jacobs frequently fields technical and not-so-technical Pathfinder questions, and his opinions often carry at least some weight with fans. Even if such opinions aren't officially official, they're often the best fans can expect.