With Cocos2D-X can I really submit my games to Steam and iOS App Store? It's commercial quality? Do you have some samples of games built on Cocos2D-X that is on sales in Steam?
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I dont think steam will bother in what you use for engine, as long as your product are stable, correct and worthy. if they find your game filld with bugs and crashes, they are probably not going to release it. Just take a look at What happend with warZ. they did use a pretty well known engine, but the game was filled with bugs and had promises it dident held up, that ended in steam removing the game. So if you dont to it like that i dont see any problem using Cocos2d. Better send them a mail and ask them.
Regarding Steam: These two threads ask the same question but have not generated any conclusive answers.
The Steam FAQ makes no mention that any particular frameworks are disallowed. In fact the SteamWorks API is C++.
Regarding the iOS App Store: There are apps for sale that use cocos2d-x, here is just one: