On a related note: How do you feel about deprecating support for JUnit3? 3.8.2 was released 20 years ago...

re Junit 3: For projects using GWT and its GwtTestcase, JUnit 3 is still the only option. But I'm not sure if any project uses GWTTestcase and PMD.
Maybe it's sensible to ignore JUnit3 in new rules but not actively remove it from existing ones (?)

Also keeping support for old codebases is aligned with the discussion in #6095 .

re Junit 5 and 6: I would prefer using JUnitJupiter over JUnit56 because I assume version 7 will also keep the naming.

One more reason GWT is weird. But yes, that's a strong argument for keeping JUnit3 support.

About the renaming for the rules: we have some rules, that generally apply to unit tests. These are now prefixed with just "UnitTest..." (e.g. https://docs.pmd-code.org/latest/pmd_rules_java_bestpractices.html#unittestassertionsshouldincludemessage) (they have been previously named https://docs.pmd-code.org/latest/pmd_rules_java_bestpractices.html#junitassertionsshouldincludemessage).
These rules apply to multiple frameworks (or in other words - we support multiple frameworks, junit3,4,5,6, testng, different assertions frameworks).

Then there are rules, that apply only to JUnit3, e.g. the naming stuff: https://docs.pmd-code.org/latest/pmd_rules_java_errorprone.html#junitspelling - there are probably some more. These rules don't make sense in a junit4 or later context or any other framework. And this rule applies to junit3 only despite the name: https://docs.pmd-code.org/latest/pmd_rules_java_bestpractices.html#junit4suitesshouldusesuiteannotation

Then there are rules, that apply only to Junit (3+4, 4 only or 5++), but not to other frameworks.

I first thought, maybe we could use the convention that whenever we use "JUnit" in a rule name, we mean JUnit Jupiter (5+6), and rename some older rules to "JUnit3" or "JUnit4" as appropriate. But after writing this up, I'm not sure, if it will be so easy...

I agree, that JUnitJuper-Prefix will be more future-proof. I have a slight preference to use a shorter "JUnit"-Prefix as a convention of the "current" generation of JUnit - just because it's shorter. But "JUnitJupiter" would be fine as well.

But maybe, what we miss is a tagging/label system for rules (see also #4298) - it would be nice, if you could find rules for unit tests first by selecting the label "unit testing" and than narrow it down to the rules, that are e.g. relevant for "junit jupiter" or so. We encapsulate this information currently in the rule name, as we don't have a different place. But adding a tag/label is a whole other story.

P.S. Renaming is possible, we have a process for it: https://docs.pmd-code.org/latest/pmd_devdocs_rule_deprecation_policy.html#renaming-rules