It definitely was the plan, but there is a huge amount of debate (and deafening silence on Frontier's part) if this is still the plan since console support was dropped and Horizons will remain as-is forever at least on XBox and PS4.
Personally, I hope not. As I said above, there's no reason I see that Horizons and Odyssey players cannot instance together in space, or even at stations (though obviously Horizons players won't be able to follow Odyssey players on foot). It's just sharing telemetry data, after all. It's even okay if Horizon players are seeing different planets than the Odyssey players in their wings from orbit, because it's just a backdrop at that point. I see different stars and planets than everyone else, and that doesn't stop me from instancing with other Horizons players.