[2024] Is it worth installing Darkness Falls without having tried vanilla Alpha 21?

It depends what counts as so many things changed. The complete release notes[7daystodie.com] for A21 feels a hundred pages long (it’s also pinned on this forum), but you can read it and decide for yourself. Magazines and water changes get the most attention. There are plenty of other features - chunk reset, infestation quests, redesigned spears, permadeath, HUD-less mode, Twitch extension, etc. - that are a big deal to some but not others.