Tested with all the versions of Proton from Experimental until 3.7-8 they all get stuck on the Loading... screen. The game is thus unplayable anymore on Linux.

It's odd that it used to work with Proton a long time ago, I've got 66 minutes playtime on this game.

Guess an update borked the game.

Note that newer versions of Proton load the initial game Loading... screen in a wine desktop window, and older versions of proton load the Loading... screen fullscreen. Game is still unplayable in both scenarios.