DF had a massive update probably a decade or more ago that changed the game from 2D to 3D (still represented as 2D z-levels though). With such a change, obviously some people would want to stick with the old version. There have been numerous large updates since then (the game has been in development for 22 years) and with each you get some people that just don’t want to update, either because it might ruin their current games or they prefer to avoid new features etc.

Another example is the various Dungeon and Dragons wikis that allow you to toggle between versions, since it has existed for 50 years now.