see
Pedantic Grognard
Yes, in Waterdeep: Dungeon of the Mad Mage, pages 250-252 and 297. Where they're explicitly established as flying in space, while not established as planar-capable.SpellJammers are already in 5ed.
Yes, and we also had it said back in Volo's Guide to Monsters that a nautiloid could go to the Astral and go between stars.A Nautiloid would not go to Avernus (from SpellJammer setting) and yet, this is what we see in Baldur's Gate.
The thing is, that's not remotely new. All the way back a bit more than three decades ago, it was established that some spelljammers could plane shift ("Pirates of Gith" entry, MC7 Monstrous Compendium Spelljammer Appendix, 1990), and that some people on the Astral used ships to get around (see "Voidjammers", Dragon #159, July 1990). That didn't merge Spelljammer and planar travel then (back before Planescape was even launched as a setting). Why would it now?
The settings can cross over some, sure, but they're fundamentally different (one's centered on Sigil and philosophical factions and the struggles between planar beings, the other is flying around in space between planets in a setting filled with elven, illithid, neogi, beholder, and unhuman ships). Imagining some ships on the Astral is a fusion of the two settings is like imagining someone in the Grand Duchy of Karameikos making a dragonlance or two is a fusion of Krynn and Mystara, or adding a magic railroad between Urik and Tyr would be a fusion of Eberron and Dark Sun.
That's why I assumed you hadn't played them both, whatever material you may own. A few cameos isn't a merger. The kind of violence that would have to be done to both settings to actually fuse them is huge.