With simplification of branches (removing distinction between short-living-branches and long-living-branches) we deprecated sonar.branch.target scanner parameter. There are other ways to achieve issue tracking between branches that might still be relevant for some users (for example in when users don't use pull requests in their workflow), so we should extend the message with docs reference.
Updated message content:
Parameter 'sonar.branch.target' passed to the scanner is no longer supported. See the Branch Analysis documentation for more information on current branch setup.