Say I have a terrain.
And I have cave
And skylight on top of it.
With current setup my cave will be lit by skylight. Even though it is big cave, and there no chance that skylight will ever reach it (in reality).
Fully occluded sky light is probably beyond realm of possibility right now.
But what about something like light volume ?
What Light Volume does, it will cut off all lighting, and instead it could provide it’s own lighting (ambient).
It’s true dynamic skylight shadowing (DFAO) has a limited shadowing distance and this will cause indoor areas to still get some sky, they don’t even have to be that large (DFAO can only cause full occlusion in about 1m).
Another method would be needed for this, something very low resolution to make the cost reasonable. The distance field level representation would still be useful to compute this sky shadowing though, since the shadowing needs to be soft.
I think you can do that with post process volume and ambient cube map… but i am not sure
You can’t. Post process won’t block any lighting, and that is main problem (;.
the cubemap would, it you swap it for your skylight (instead of using a scene capture for it). still it would involve a hard switch.
personally I thought the scene capture reflection probes could be usable as skylight ambient color (providing “local” ambient light as captured from multiple points instead of one source) but I was disappointed to find it’s not the case 
I’d love to know how titles like MGS5, Assassin’s Creed & Far Cry handle their fully dynamic lighting and what limitations their system has.
I think what the majority of users would like is a single, solid solution for dynamic lighting, bounce, AO, shadows, sky, fog & ToD that is visually and functionally competitive with most cross-platform next gen games out there (see the above 3 examples), but doesn’t have to be cutting edge. Even if that came with limitations of (no more than 3 lights overlapping per pixel, only 1 bounce per light, limited shadow resolution). And then more optional advanced systems for expert users to fiddle with.
Instead, new users are stuck having to juggle 3 types of lights, light maps, Lightmass itself, optional cascade shadow map distance settings, different GI options, different AO options and the new ray cast shadows.