sub-directories for different versions.
Link to the appropriate version in the release’s release notes.
Have a page in the wiki that links to older versions.
The benefits to this approach are:
-predictable URLs that anyone can change in their URL bar
-releases (from now on) will always have valid link to their javadoc, forever
The only downside is using the storage space, really.
Edit: that being said, note that I do not do this for my own projects… only the latest is on the web. Each release gets a javadoc jar on bintray anyway. So at any time people can get the javadoc for any versions.
So there is precedent for not doing it the way I suggested above.