Yea, forward rendering is the proper engine topic.
Since it changes the whole rendering pipeline to forward shading.
Its not really “just a tick box”, some material things you might have to re-do.
Shadows may look worse, fog etc may be gone, temporal AA may be removed, etc…
I believe you loose transparent materials as well… (then again transparency on mobile is an horrible idea 
)
For mobile releases it may just put you in a spot where you can test and release witout burning the device up.
When you tick the box all the shaders will have to rebuild. You may even have to restart the engine.
Be prepared to wait a couple hours…