One simple question:
Will deprecated methods ever be removed from classes and can I remove them if I want to commit some new stuff?
Deprecating method is a way to avoid to code breaking, where tell us that there’s a better method. So is recommended to keep them alive…
Deprecated methods will be removed one major release after they have been marked deprecated (e.g. 3.0 → 3.1, beta->release etc)
Ok, another one:
Do I have to provide functioning for deprecated methods if I want to change stuff? Or can I just leave them blank if no similar functioning is going to be used? 
Can you be more specific? I think otherwise the answer is “it depends”.
Well I kinda wanna make particles engine more usable, by for example making all the changes to the particles go through influencers. So one emitter would not have only one possible influencer, but maybe more. There would be color influencer, where you would define at what time and how would the color of the particle change, then for example size influencer, motion influencer, gravity influencer, image influencer and so on. User would just have to instantiate those provided influencers, set them and add to emitter. But there are lots of methods which deal with these values directly now, since they are stored in emitter class. What should I do with such methods?
setAssetManager is marked as Deprecated… What is the reason?
I want to have the opportunity for headless applications. By loading a copy of game maps on the server with a single AssetManager
I imagine because changing an asset manager at runtime is dangerous.
Can you explain more why being able to set the asset manager somehow facilitates something on the server?