zerodin: Seems to be free for everyone who bought the older versions on anything(except mobile)
I can confirm that free copies were waiting for me on GOG, Steam and my Switch.
DataCorrupter: How do I get this free? I have bought Doom 2 a couple of times on CD. My buddy borrowed it and I didn't get it back, but it was years later that I realized it. I still have it on my shelf I think. I think I had Doom 1 on floppies, but those are long gone.
It's not for retail releases, it's only for those who purchased the game digitally through GOG and Steam for Doom and/or Doom II as well as on consoles and the Epic Game Store for those who had the previous Unity ports.