this is a good point I missed. A lot of eberron stuff has been updated & republished for 5e, I probably still use some of the 3.5 eberron stuff for lore & such though & have used the 3e ravenloft book in the past too.A ton. Source material and modules are and always will be valid. I'm borrowing heavily from Raveloft source material for running CoS soon. I consistently go back to Greyhawk and FR source material as well. I dug out my old Greyhawk stuff when I was brainstorming on a Saltmarsh campaign and was recently reading up on Myth Drannor.
For stats/mechanics, I don't use much from 1ed/2ed, except maybe salvaging ideas from old-school monsters. 3ed still has value for mechanics because it is d20-based. The classes and prestige classes can be used for homebrew.