I'm too new to know first hand, but according to
a comment from @jojon:
In addition to all of those, Scorpion SRV and Colonia on-foot engineers. Dockable megaships didn't get a walkable interior until a few updates in either, though sooner than FCs.
Broadly, I think it's best to view things as:
Updates 1-12: actual Odyssey updates. Mainly bugfixes and polish, but some extra content additions too.
Update 13-18: "Aftermath". Thargoid war content which is mostly using spaceships, which includes a few on-foot features.
Update 19-??: "Ascendancy". Powerplay content which they've suggested will also have some on-foot features but mostly use spaceships.
Looking further back, there weren't
that many Horizons-specific features added after the 2.4 release was done - a few more types of Guardian sites, some new ships with a Horizons-only tag, a few story/mystery POIs ... but most of the content added in Beyond didn't require Horizons, for one reason or another. With Odyssey, they said that they were moving to a "develop-then-release" model rather than the "release-then-develop" of Horizons, so it probably should be expected that there weren't any further
major additions to it planned.