How to not fail on deprecation warnings

I believe that the behaviour comes because of this line in the core phpunit.xml.dist: https://git.drupalcode.org/project/drupal/-/blob/11.x/core/phpunit.xml.d...
failOnWarning="true"

You'd need to override that value to false for your module somehow.

Given that the core's default value is set to fail and that each module can add their own phpunit.xml(.dist) file, I'm closing this as "Work as designed".

@drunken monkey If the aim is to see the deprecations but not make your pipeline fail, then an alternative if you don't want your project to have its own phpunit.xml(.dist) as mentioned in #3, is to allow the pipeline job to end with an amber warning not a red failure

phpunit:
  allow_failure: true

Even with the warning the overall status of the pipeline will be green (providing there are no other red failures).

The downside of this is that real test failures would also be classed and 'OK' and not make the pipeline fail. So another way would be to leave your 'current' phpunit without showing deprecations, but use OPT_IN_TEST_NEXT_MINOR:1 and show the deprecations there instead. All of the 'next minor' variants are allowed to fail so you do not need to specify that in the job.