The various incarnations of Doom have been available under a variety of software licences.
The alpha releases of doom carried various warnings about leaks, including the following in the Doom 0.4 menu:
HEY,MAN-IF THIS GETS OUT, YOU CAN ALL KISS YOUR AUTOGRAPHED DOOM HINTBOOKS GOODBYE.(c)1993 id Software
The alphas and press release version of doom carry the following disclaimer in their ENDOOM lumps:
This is the alpha version of DOOM. If you are not a beta tester, then you are quite the bad person. Delete your copy of DOOM right now and you will be safe from our wrath.
Doom 2 was the first traditionally commercial release of doom. It was first sold boxed in high street stores on September 30, 1994 and no shareware or demo version was released.
Source code release
ID Software released a modified version of the Doom source code to the public on December 23, 1997. This release was under the DOOM Source Code License, described within the file DOOMLIC.TXT.
This licence permitted use the source code for educational purposes only. It explicitly forbid selling the source code or using it for commercial gain as well as distributing the source code.
After the loss of the source code to glDoom in 1998 from a hard drive crash, some people, including John Carmack, suggested that if the author had been required to distribute the changes he made to the code, then it would have been more likely that others would have had copies of the source. As a result, id Software relicensed the source code to Doom under the terms of the GPL on October 3, 1999. Many existing source ports followed suit by obtaining permission from their copyright-holders to relicense their work.