Wot this lot is Andys recent Slann off of FB
can be seen here? And David Woods natty red ones (sideways photo
here?
Space Slann started off in WFB2 as "High Age" Slann - as they were the first intelligent settlers of the Warhammer world, and subsequently had space ships, ray-guns and whatnot in the prehistory of the setting. By the "now" of the timeline, the Slann had fallen from power, broken the warp gates, allowed raw Chaos into the world, and only the Amazons remembered how to maintain the alien tech, When Rick wrote 40k he left some hints at this, but eventually their centrality in the space version mythos got shifted to the Eldar.
So if you wanted to game earliest wars in the Warhammer setting, say Slann vs. rebelling Orc slaves, then Space Spann are a good start. IMHO that might be where GW heads after The End Times. Not with the WFB2E timeline, but with campaign sets / army updates at different points in the warhammer story (War of the Beard, Sigmar clearing out the Old World of Goblins, etc.) anyway, that's not Oldhammer!
As for why they got dropped, it's almost certainly that the sales didn't warrant their development. Space Marines sold by the boatload, Space Orks, Space Elves, Chaos, Genestealers (helped no end by Space Hulk). Imperial Guard sold. Space Slann, Space Dwarves, IG Beastmen didn't. The great simplification of the setting into cartoon caricatures of itself in 40K:2E was almost entirely commercial decisions to streamline the range.