Is religion a good forward thinking plan to reduce population growth if that religion teaches that at some point masses of people will be killed, and that this life values is less that one to come? Or if that religion teaches us that this life is merely an illusion, not real, like the film the Matrix?

Um,...no.  If some religion, any religion, teaches this type of doctrine, it is patently evil.

The major monotheisms all endeavor to cheapen this brief, but one certain mortal existence with the fanciful, yet groundless promise of another better one to come. Life as a dry run. A claim void of any logic or proof beyond wishful thinking. Not among qualities one would seek to include in an effective plan, forward thinking or otherwise.

If this life is an illusion, a fatuous claim I confer no support to, then what can we say about an [alleged] afterlife, for which the only basis is faulty human conjecture, asserted during the course of said illusion?

Again ... Your question can be answered by looking at history.

There have been no religions, that I am aware of, where the people voluntarily submitted to slowing population growth - or committing suicide.  >  In the instances where there has been mass suicide, it has been for different reasons.  And is more commonly mass murder, as the followers are tricked into killing themselves.

Religion had its place and time. Because of it, many people have such extreme disagreements as to murder those whom do not agree with their particular brand of belief system.

The only thing it is good for, in my humble opinion, is family tradition in preserving a small bit of culture. The rest of it where you force a belief system unto innocent children can be thrown away.