Use these links to access the API docs of the various Sling versions:
These apply to the "big" Sling releases, but in between those we often release individual modules, including their version-specific javadocs.
javadocs.io also provides archived javadocs for all Sling modules, and actually for all open source software hosted at Maven Central.
To get the javadocs of a particular module there, use an URL like http://javadoc.io/doc/{org}/{artifact}/{version}
, like for example http://www.javadoc.io/doc/org.apache.sling/org.apache.sling.api/2.9.0