Games like Project Threshold
Games like Project Threshold include Still Wakes the Deep, Outlast, ROUTINE, Mouthwashing, and Layers of Fear (2016). We matched them by the tags players give Project Threshold on Steam (Adventure, Walking Simulator, 3D, Cinematic, and First-Person): closest matches first, and among equally close ones the better-known games.

Still Wakes the Deep
Shared: Horror · Walking Simulator · Psychological Horror

Outlast
Shared: First-Person · Horror · Psychological Horror

ROUTINE
Shared: Adventure · Cinematic · Psychological Horror

Mouthwashing
Shared: Horror · Psychological Horror · Walking Simulator

Layers of Fear (2016)
Shared: Walking Simulator · Horror · Psychological Horror

Firewatch
Shared: Adventure · Walking Simulator · Atmospheric

SOMA
Shared: Horror · Psychological Horror · Atmospheric

KARMA: The Dark World
Shared: Walking Simulator · First-Person · Horror

Blair Witch
Shared: Adventure · Horror · First-Person

MiSide
Shared: Psychological Horror · First-Person · Horror

Summer of '58
Shared: Walking Simulator · Horror · First-Person

The Cabin Factory
Shared: Walking Simulator · Horror · Psychological Horror

Welcome to Kowloon
Shared: Adventure · Walking Simulator · Horror

Fears to Fathom - Ironbark Lookout
Shared: Horror · Adventure · Psychological Horror

The Exit 8
Shared: Walking Simulator · Horror · 3D

What Remains of Edith Finch
Shared: Walking Simulator · Adventure · First-Person

Scanner Sombre
Shared: Walking Simulator · Adventure · Horror

Scorn
Shared: Horror · First-Person · Atmospheric

Lethal Company
Shared: First-Person · Horror · Psychological Horror

No, I'm not a Human
Shared: Horror · Psychological Horror · Atmospheric

The Stanley Parable
Shared: Walking Simulator · First-Person · Adventure

The Beginner's Guide
Shared: Walking Simulator · First-Person · Adventure

POOLS
Shared: Walking Simulator · Psychological Horror · First-Person

Garten of Banban
Shared: Horror · First-Person · Psychological Horror