Planes and planets

Although space travel seems fairly unpopular in OGL fantasy gaming, it has appeared in surprisingly diverse forms.

Portal travel only. By default, most fantasy campaign settings limit themselves to a single planet. If multiple planets are introduced, then they will generally be reachable only through portal or by traveling through one of the transitive planes. (WarCraft is a popular example.) Spacecraft either don't exist or are so rare that the PCs won't be expected to ride inside one. This model seems to be the most common for settings that involve multiple worlds, such as Golarion's Distant Worlds supplement, presumably because space travel occupies a lot more conceptual space.

Science fantasy. When space travel gets involved as a standard fixture of the setting, the most common manifestation is what some label the "science fantasy" subgenre. (Star Wars is a popular example.) What I don't like about this model is that tabletop settings, like Dragonstar, Starfinder, or Starjammer, typically sloppily throw scifi and fantasy conventions into a blender with only the clumsiest attempts to integrate them. Magic and technology exist side-by-side, operating on completely different rules.

Magitech space fantasy. However, some settings don't lazily throw scifi tech into a standard fantasy setting and pretend it works. Instead, they might devise magitech spacecraft to match the fantasy setting. Campaign settings like Shadow of the Spider Moon, Aether & Flux, Aethera, and Necropunk are examples. They rely on technology powered by magic or alternate physical laws, sometimes having multiple competing methods.

Retro space fantasy. The science fantasy and space fantasy subgenres still assume that space operates like it does in reality: explosive decompression, empty vacuum, etc. However, some settings go even further with the fantastical elements, jettisoning real physics entirely. Spelljammer introduces breathing in space, crystal spheres, phlogiston, geocentric systems, and so forth. This model is recycled by some OSR settings like Voidspanners and Dark Dungeons.

Planes as planets. The models I mentioned so far still assume that space travel is limited to the "material plane" (as D&D calls it) and that planar travel operates by an entirely separate subsystem. However, that doesn't have to be the case either. 3pp like Classic Play: The Book of the Planes and Blood & Treasure suggest that the planes could be arranged in the same way as planets and visited through fantastical means of space travel. At this point, there is no distinction between space travel and planar travel.


ADDENDUM 11/19/2019: Honorable mentions to Aetherial Adventures and Cabbages & Kings: The Steampunk Renaissance.