Astral plane and Astral Sea are the same thing.
And, I think? they always were.
Astral Plane
Space.. and the Astral sea/plane were not, and at some point, did get merged.. I think. Im no historian here.
Spelljammer - Wikipedia
AD&D spelljammer has gravity, space, etc. Its space, not the astral sea..
But, by 4e, Gith, Illithids, Spelljammer, etc are all in the astral sea.
The Plane Above: Secrets of the Astral Sea (4e) - Wizards of the Coast | Dungeons & Dragons 4e | Dungeons & Dragons 4e | Dungeon Masters Guild
probably directly related to this. Although, I seem to remember thinking Gith were Astral sea back in 3.5e... so. im not sure.
gith article leads to this.
Wildspace
Realmspace
Phlogiston
this is what "space" is now it seems.
Currently, instead of many material planes for each Setting. There is ONE material plane, and each Setting is a solar system housed inside a crystal sphere.