2016-12-07 (updated 2016-12-07)
"Doom" is not a word in the Webster's dictionary anymore. It is in fact owned by Zenimax who sued Webster's and the people behind DoomRL for using the word. Which is why it is "DRL" now.

Zenimax took this action seemingly because one of the developers of DoomRL created an unrelated roguelike called "Jupiter Hell" and solicited investors on Kickstarter. And Webster's because they sell dictionaries.