The reality is that some people prefer other systems and as a result try to say horror isn’t possible in this system... it mystifies me why they can’t acknowledge both.
There are also attempts here to try and apply a very narrow and clearly non-standard definition of horror to try and justify why horror isn’t possible in D&D.
- Claims the PCs can’t be competent
- Claims the PCs can’t win
- Claims the adventures have to be futile/insignificant
- Claims horror will be ruined by spells, or magic items, or turn undead... or even barbarian rage
- Claims the rules can’t be adjusted (even when there are written optional rules)
- Claims that sanity mechanics are needed (even though they exist optionally in D&D)
Do not listen to these claims, they are simply not true for any normal definition of horror. Don’t listen to people who believe they can tell you the ‘true’ form of D&D.
Nobody who is interested in giving a 5e horror campaign a go (possibly because that’s their groups go-to system) should be put off by what’s been claimed in this thread. There are lots of great tips from people here to build a horror campaign.. As I said, our horror campaign was the most satisfying i have ran.
So, I've read your posts on how you do horror in 5e, and they're good advice but have almost nothing at all to do with using the 5e system to do horror. Most of it is entirely system agnostic. The points you do make that touch on the 5e system are all about how to change that system to facilitate horror, which appears to implicitly acknowledge the premise of the OP. Can you make an argument that shows how specific 5e mechanics lend themselves to enabling horror? That would be the argument that makes your point, not that mood lighting, good soundscapes, and use of system agnostic tropes make for good horror.
To do the opposite, and make the mechanical case against horror, I'll point out what's already been said about PC's power levels being difficult to challenge, but I'll focus on the combat engine more closely. The 5e combat engine is a detail oriented, systemic approach to combat. It breaks a combat into clean, clear chunks where things are rather clinically resolved. Sure, you can punch up the descriptions, but you have to do this because the combat engine forces it. Because the combat system is so detail oriented, and outcomes are limited to those the engine produces (largely status effects and hp loss), you don't have a great breadth of either stakes or outcomes to play with. To generate scary monsters you have to make them very powerful relative to the PCs so that hits are dangerous, and you have to make them resistant to things the PCs have. This, within the engine, tends to generate frustration in the players because you have to go through the steps even when it's clear the numbers are weighted against them. This deflates tension and erodes the horror feeling by becoming mechanical, even with good narration. You can ignore the combat engine, if you want, but this, again, admits that the 5e system doesn't do horror well.
None of the above is to say that you cannot do horror in 5e, just that the nature of the system fights you when you do. You can use props and mood setting at the table to help overcome the system, as well as good pacing and use of horror tropes in play to also overcome the system's resistance. That you can do it doesn't actually show that 5e is a good system for horror or that the way that the system is structured doesn't actively act to counter horror in many regards. It just says that you've done the work and overcome the system. I do this quite often in my 5e games -- I like a lot of eldritch horror in my games so they all tend to get parts of it -- but I do it knowing how the system works and then work around that. Some of the other games I run have systems that don't get in the way of horror, and it's easier to lean into the horror tropes because the system enables them rather than fights against them. This is, largely, what the OP is talking about -- not if you can run a horror game in 5e, but if 5e's system works for horror or fights against it.