Population projection is, however, tricky. Unless we can establish that an authority has made projections and relied upon them for decision making, it will be an 'opinion' which is not covered by the definition of information under RTI Act.
Slum population is also difficult but we have specific departments in MCGM and Maharashtra Government for rehabilitation of people in slums. Therefore, you may approach these and find out if they have any actual data/ projections.
There is every chance the SRA authority, MMRDA, MCGM, Maharashtra government zopadpatti nirmulan vibhag etc will give you different definition of slum, different period for which they have actual data, different methodology to make projections, etc etc.
The information you want is so politically combustible, you are not likely to get an answer easily. However, it will be an interesting to see how each organisation behaves when faced with a simple (!) question.
If you plan to use RTI act to get this information, please go thru RTI Guide to learn the technicalities of paying RTI fee, finding the relevent PIO, etc.