Games like Loop 9
Games like Loop 9 include No, I'm not a Human, MiSide, Poppy Playtime, Outlast, and Layers of Fear (2016). We matched them by the tags players give Loop 9 on Steam (Horror, Psychological Horror, First-Person, Choices Matter, and Investigation): closest matches first, and among equally close ones the better-known games.

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

MiSide
Shared: Psychological Horror · Horror · First-Person

Poppy Playtime
Shared: Horror · First-Person · Singleplayer

Outlast
Shared: Horror · First-Person · Psychological Horror

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

Firewatch
Shared: Atmospheric · First-Person · Adventure

SOMA
Shared: Horror · Psychological Horror · Atmospheric

What Remains of Edith Finch
Shared: First-Person · Atmospheric · Story Rich

Sally Face - Episode One
Shared: Psychological Horror · Horror · Story Rich

Little Nightmares II
Shared: Horror · Psychological Horror · Singleplayer

Little Nightmares
Shared: Horror · Atmospheric · Singleplayer

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

Resident Evil 7 Biohazard
Shared: Horror · First-Person · Singleplayer

Phasmophobia
Shared: Horror · Psychological Horror · First-Person

The Casting of Frank Stone™
Shared: Horror · Choices Matter · Psychological Horror

Outlast 2
Shared: Horror · Psychological Horror · First-Person

Subliminal
Shared: Psychological Horror · Horror · Atmospheric

Bendy and the Ink Machine
Shared: Horror · First-Person · Singleplayer

The Test
Shared: Psychological Horror · Multiple Endings · Choices Matter

Outer Wilds
Shared: Atmospheric · Adventure · Mystery

Oxenfree
Shared: Choices Matter · Atmospheric · Horror

Amnesia: A Machine For Pigs
Shared: Horror · First-Person · Psychological Horror

Five Nights at Freddy's: Security Breach
Shared: Horror · First-Person · Singleplayer

SILENT HILL 2
Shared: Horror · Psychological Horror · Atmospheric