A couple years earlier than your request, but I think Timecrimes (2007) is underappreciated.

I don't necessarily know how deep the deep cut has to be,, but I'm going to say The Void (2016).

Not sure how many of these count as deep cuts, but here are a few from the last decade (or so) that I enjoyed:

Pontypool-Actually came out in 2008, but it's still worth watching, especially if you are looking for a zombie film that does something different. A washed-up radio DJ listens from his booth as his town breaks down around him following an outbreak of a mysterious infection. It definitely leans more toward psychological horror rather than traditional zombie horror, so I could see someone complaining that it's slow and doesn't really show anything until near the end.

Butterfly Kisses- A found footage film about a film student investigating a found footage film. Maybe a bit too meta for some, but I enjoy a found-footage movie that doesn't take itself too seriously. It seeds just enough doubt about whether the footage is "real" that it leaves you guessing how it's going to end.

Triangle-2009 film about a group of friends who come across a seemingly abandoned ocean liner while out on the water and decide to explore it. Maybe not that deep a cut, but it's worth checking out if you've never seen it.

Coherence- Not actually sure if this counts as horror. I see it classed as a sci-fi thriller a lot, but I found it while searching for something similar to Triangle. A group of friends gets together for a dinner party only to notice strange things happening after watching a comet passing overhead. It's hard to say anymore without spoiling it. Another one that could be seen as slow and without a lot of "real" scares. Still might be worth checking out if you want something that is slow-burn and more psychological.

@CharlesDarwin I thought "Deadstream" was really good, and I liked "Hell House LLC" and "The Dark and the Wicked."

I don't know how deep these might be, but I'd suggest Vivarium, Landmine Goes Click, and I Saw the Devil. Those are from 3 very different horror subgenres so hopefully at least one is appealing if you haven't seen them already.

Since I haven't seen them mentioned, I recommend:

• Leaving D.C.
• Savageland
• Be My Cat: A Film for Anne
• A Record of Sweet Murder
• We Are Still Here
• Head Count
• Better Watch Out
• Antrum
• Historia de lo Oculto
• Anything for Jackson
• Alone (2020)
• Come True
• Soft & Quiet