Where to get Docs of old Spigot versions?

you can just compile them yourself using maven, spigot only hosts the most recent build of the javadocs.
just clone the bukkit project: git clone https://hub.spigotmc.org/stash/scm/spigot/bukkit.git
checkout whatever commit you need: git checkout HASH
and run maven: mvn javadoc:javadoc

 

mmmh, it couldn't resolve some references, no idea why. that means some links may be broken, but it should be good enough.