edit: the libsteam_api.so from the previous version does not work
edit2: libsteam_api.so from this package works https://github.com/rlabrecque/Steamworks.NET/releases/download/15.0.1/Steamworks... YAYKS
LISTEN GUYS, LISTEN ! I managed to make the 1.31 work under linux. There are two files missing in the package.
steam_appid.txt with content 1782210
just create this file (in your favourite text editor) in the root of your game and put said number on the first line.
Then there is missing 'CrabGameLinux_Data/Plugins/libsteam_api.so'. This file was part of the previous version (1.29) so copy it over from there, but I guess it could be found on the internet somewhere if you don't have the package anymore.