Not sure if this is a case of someone not following Deprecating or removing a Plugin or that's just the user experience ...
See StackOveflow question: pmd plugin is not available in jenkins for context.
User looking for "PMD plugin.
Search in Plugins site returns two plugins: Violations and Report Info. User obviously knows it exists but can't find it.
Checking GitHub (if you know the repo, you find jenkinsci/pmd-plugin exists. It is tagged as "Deprecated", repo is "Archived".
If you click on the link in GitHub, back to plugins site (plugins.jenkins.io/pmd/), you are presented with "403 Forbidden (nginx/1.17.7)".
From a user experience perspective, if you assume the plugin exists, from prior knowledge or external referral, you are left confused.