I know this thread is pretty old, but wanted to share my experience. To get Plutonium working for me in Linux (Arch), all I did was download the exe and added it to Steam as a non-Steam game. Once I did that, I downloaded my CoD MP titles from Steam (using Proton Experimental as my compatibility). I created a /home/myUser/Games/Plutonium directory and inside that directory, I symlinked the CoD directories that Steam created from the CoD MP downloads. I did this because the Plutonium Setup would not allow me to enter the .steam directory in my home for, but once I created a directory and symlinks that the Plutonium exe could view, Setup was a breeze. I then added the Plutonium exe to my Application Manager (in KDE) so that I could add the Plutonium launcher icon on my task bar.
Doing it this way, eliminates the need to use Wine or Lutris and was honestly, the easiest setup I have had for getting Plutonium to work on Linux. Hope this helps anyone who finds it!