There's a few things that I can see that blur the line between placeholder and work-in-progress, assuming there is a distinction.
Notably, the BGS I see as being very much incomplete. They have done a bit more work on it in Beyond, but it's still a far cry from being capable of standing on its own without constant fiddling and developer intervention.
Similarly, the Thargoid war is mostly just a layering of MVPs in order to give the illusion of "something" happening, rather than a coherent set of actual mechanics that flow properly together and simulate a full response from humanity. This may be partially because the BGS isn't yet capable of simulating widespread conflicts though, it can barely even give the impression of two minor factions squabbling, let alone simulating the full political, military and economic webs that exist between Aegis, its fellow naval factions out at Maia, the three superpowers and the Thargoid menace.
There's a few other things though that just feel like "proof of concept", an initial draft that got something out of the door and would definitely pass for an early Alpha product, but have received very little attention since. They don't feel like they are just there to mark a future feature, but instead they are the abandoned seedling that has the potential to bloom into a feature.
Notably in this category is NPC crew. Sure, they can fly your SLFs and look after your ship if you want to control the fighters yourself for a bit, but that's literally all they can do. They can't even fly your ship or the fighters while you are on the surface in an SRV, let alone do things like operate SRVs themselves to give you more support on the ground (or to get the stuff for you after being dropped off planetside while you provide the air support). To take things further, they are completely incapable of doing anything outside combat, such as helping you scan things or picking up messages and tip-offs for you.
The planetside stuff also feels very much like an early Alpha product. Sure, it's playable and shows off some interesting concepts and potential, but fundamentally there isn't much to it. More stuff to do, more things to interact with, more vehicles to use and generally integrating it with more of the rest of the game would go a long way to making it feel like a proper feature rather than just a bolt-on proof of concept.
For actual placeholder mechanics, there's a few that catch my eye. Notably, RESs feel very much like a placeholder, although I would have no issues if they were to be simply renamed to "pirate raid in progress" and the HAZRES being "pirate jamming detected" to give a better idea of what they are actually about. Community goals feel very placeholderish, as they feel like they are just there to shore up current shortcomings in the BGS rather than a feature in their own right. Powerplay I'm not sure about, as it is definitely unfinished but I don't know whether I feel like it is just an incomplete and abandoned early-Alpha feature or whether it is just there to show off the Powers. There's probably others if I bothered to think about them for a bit more.