As noted by jglick and danielbeck here and here if a plugin uses a custom action.jelly for a RootAction (or an Action that is also displayed for the main dashboard) then the action may no longer be shown, may be shown when it should be hidden, or may be shown but incorrectly.

For example, a plugin may choose to hide itself in the jelly based on a permission (rather than doing this in the Action code. this is the case of the purgeBuildQueueAction https://github.com/jenkinsci/purge-build-queue-plugin/blob/adeca5c614fe060ce9960dce52399c27d6abf64f/src/main/resources/io/jenkins/plugins/PurgeBuildQueueAction/action.jelly where it is not longer displayed anywhere.

We have other custom implementations that do provide an Icon and a custom action.jelly. Previously these where hidden by the Jelly, however now they show up unconditionally in the header bar.

Some of these actions may be destructive (such as the purge build queue, and may need to use a HTTP post and require a confirmation). this functionality appears to be lost due to the ignoring of the action.jelly.