It's a WIP (Work in Progress).
If / when it's finished,
probably it will be added to GC4, but no guarantee, I will be discussing with
@micdoodle8.
Until now each new dimension has meant the launch of a new Galacticraft (Mars = GC2, Asteroids = GC3, Venus = GC4) but if we add Deep Space we will be breaking from that rule and adding a new dimension to GC4 after GC4 is already released. Seems OK. It's only a
small dimension, it's not a whole planet.
Please do not anticipate too much. It is possible that there will still be some problem with the Deep Space dimension - e.g. incompatibility with other important mods - or we will decide not to release it at all.
It is not currently in the game (not even the dev versions) so there's no way for players to access it.