Games like Flat 9
Games like Flat 9 include Platform 8, Poppy Playtime, The Exit 8, Layers of Fear (2016), and Visage. We matched them by the tags players give Flat 9 on Steam (Indie, Walking Simulator, Horror, First-Person, and Simulation): closest matches first, and among equally close ones the better-known games.

Platform 8
Shared: Walking Simulator · Adventure · Simulation

Poppy Playtime
Shared: Horror · First-Person · Indie

The Exit 8
Shared: Walking Simulator · Horror · First-Person

Layers of Fear (2016)
Shared: Horror · Walking Simulator · First-Person

Visage
Shared: Horror · First-Person · Psychological Horror

The Death | Thần Trùng
Shared: Horror · Indie · Psychological Horror

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

Buckshot Roulette
Shared: Horror · First-Person · Indie

MiSide
Shared: Horror · Psychological Horror · First-Person

Blair Witch
Shared: Horror · Indie · Adventure

Outlast
Shared: Horror · First-Person · Psychological Horror

Among the Sleep - Enhanced Edition
Shared: Indie · Horror · First-Person

Dollmare
Shared: Horror · Simulation · First-Person

From The Darkness
Shared: Horror · Indie · First-Person

[Chillas Art] The Convenience Store
Shared: Indie · Horror · Simulation

Scanner Sombre
Shared: Walking Simulator · Horror · Indie

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

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

Slender: The Arrival
Shared: Horror · Indie · First-Person

Firewatch
Shared: Walking Simulator · Adventure · First-Person

Welcome to Kowloon
Shared: Horror · Walking Simulator · First-Person

Still Wakes the Deep
Shared: Horror · Walking Simulator · First-Person

Garten of Banban 2
Shared: Horror · First-Person · Psychological Horror

Ready or Not
Shared: Realistic · Horror · First-Person