Placeholder issue to track changes to the ruleset schema, for which we could prepare on master by issuing deprecation warnings/ support new syntax for forward compatibility

• [core] Deprecate missing language attribute on rule definition #2617
• [core] Deprecate <lang>-<ruleset> hyphen notation for ruleset references #2352

Related issues

• [core] Select XPath version in violationSuppressXPath #1676

Remaining tasks

Probably for PMD 7

• [core] Remove support for <lang>-<ruleset> hyphen notation for ruleset references #4313
• [core] Remove ruleset compatibility filter (RuleSetFactoryCompatibility) and CLI option --no-ruleset-compatibility #4314