Sadly I have to view it as DRM... since it prevents me from playing with my friend on the same network segment even if we're some where there's no internet or our internet is offline (which happens frequently despite game companies claiming it doesn't).
For now it hasn't been an issue since we bought the 2 pack on Steam but I was interested in purchasing the GOG edition for the bottomless healing potion until I learned it was still behind the DRM wall.
I posted a question to the Steam boards as well about how the treasure keys (which are verified online behind the DRM wall) are encoded into the saved games. No response so far.
I'm a huge fan of any RPGs & am even still working on Diablo 2 & have Diablo 1 installed with the saved game of my final character. In fact I typically use Gamesave Manager
to continue my games across years if the game supports it. (I have saved game archives back to 2007.) So it's a big deal if a treasure key can cause my saved game to become unplayable if there's no internet.
This is because my ISP is unreliable & the maximum amount of time I've had to go with out internet (this year) has been 4 days.
Then you will be able to play when the local coop is available on the same machine.
The bottomless healing potion is not behind a DRM, it's just a normal item which drops in the game but you can get it instantly this way as a present.
The treasure key changes nothing and gives you items that are already in the game.