Store Page:
https://store.steampowered.com/app/477160/Human_Fall_Flat/

Game got updated with Linux support on original release in 2016.

Two years later Linux support has been discontinued. By the words of developers team was "too small to support PC Mac and Linux versions". So Linux version been discontinued. No refunds been provided to linux users. Instead developers offered Linux customers to keep playing older Linux version, leaving them with no updates.

Devs "appreciate frustration of community".

Source:
https://gtm.steamproxy.vip/app/477160/discussions/0/1735468693704602016/?ctp=5#c1735469327933155260

While having singleplayer mode, game has strong focus on multiplayer co-op. Game got number of big content updates since then. Game uses Unity engine, which has support for Linux.

Bonus:

From the FAQ of the official game website :

"I have a Mac/Linux, can I play?

Mac and Linux versions are available and are first-class citizens!"

http://www.nobrakesgames.com/human/