The last release listed in the changelog here is Version 2.1 (2016 May 20):
https://wiki.jenkins-ci.org/display/JENKINS/Swarm+Plugin

Since then 2.2, 2.3, and 3.0 have been released.

There is no changelog file for the project in github:
https://github.com/jenkinsci/swarm-plugin

That means there is no easy way to know what has been changed or fixed in the later releases.