From DoomWiki.org

I'm grateful for the two landmark decisions made by the Id guys long ago:

  1. Creating the WAD format so that fans could easily modify the game
  2. Releasing the engine source code after the revenue dried up

Because of this, and the work of many hobbyists, I've had lots of enjoyment playing classic Doom with my thoroughly-customized, EVP-based GZDoom. (I first played this game in 2020 after seeing cool gameplay mod videos on YouTube.)

Since EVP is written in ZScript, I changed a number of things to my liking. However, the Id guys did great work on the original games, so I preserved the fast, fluid vanilla player movement that I really like, as well as most facets of vanilla combat.

A nice bonus is being able to enjoy watching demos, since the gameplay is similar enough. The speedrunners are a wee bit better than me, though :)