I have met several IDs, some suggested to plaster the walls to make it even, some say it'll still crack. 

I read online that you are supposed to remove the existing paint for bondability before applying Gypsum plaster on walls. Is it true that nobody treats or strip the wall before applying the plaster??

I don't know who's telling the truth.

To plaster or not to plaster???

It is best to remove the paintwork on the existing wall before applying plaster.  Smooth wall with paint is not advisable to plaster.  After a few years, the plaster will crack and thin layer of plaster will detached from the wall. 

i would prefer them to remove the paint but it seems that they don't do it unless it is peeling, if not they'll just plaster directly on it.

In my opinion its just better to remove the paint and then plaster it