Of course they can - the first Community Goal which required the Horizons as opposed to Basic client to complete it (back when the split was 2.0 versus 1.5) was just two weeks after the 2.0 release. Barnacles - narrative content requiring Horizons - followed not long after, and there was plenty more before the "everyone gets Horizons" merge.
community goals requiring horizons i thought was a mistake due to the nature of the item involved in the community goal. But i haven't reviewed the details of that recently.
Only visiting the barnacle was out of bounds for non-horizons players. everything up to that was not. I think you could even spot them from the orbital exclusion zone ...but that might be stretching participation. I'm not convinced that was intentionally left out of bounds by fdev or if they figured (incorrectly as often is the case) that it would take longer for players to find them. But even if that's not the case, the content on the surfaces are not stopping players who can't see the surface from participating in the narrative as there are plenty of non-surface aspects.
This differs from a "no content provided" for 3.8 situation. People can still buy horizons and some players will do so under the guise of what horizons is shown as being like and performing like. If that sets them in the 3.8 version then they're liable to not be very happy to find out that they just purchased a game that looks on the surface to be active but only for a version they can't use.
Why do you think they wouldn't be? They need to keep it running on a "critical fixes only" basis for consoles anyway, but there's nothing they've said which suggests any intent for them to do more than that, and no way to realise the development time savings otherwise.
If there is no narrative pushed to 3.8, no content updates to bring gameplay in line with the rest of the game (so you dont have two game rule sets), then you aren't really getting the game they're selling. Your game has ended and you're playing some stripped down version of it.
And So long as 3.8 exists as updating the same BGS the rest of us are playing in, they wont realize development savings anyway. It's still going to be just as much overhead handling multiple branches that are becoming more and more incompatible even if they only push critical updates to them. The only way they realize savings is eliminating the disjointed branches.
3.8 vs 4.0 is being talked about like a choice, but the implementation of it being an option says fdev believes it's not choice-y enough for a large number of players to warrant continuing to spend more money supporting both branches.
