PHPUnit 9 isn't actually that widely used yet, because of its aggressive dropping of PHP versions. PHPUnit 8, 7, 6 and 5 still have more monthly downloads (each, separately!)

Source: https://packagist.org/packages/phpunit/phpunit/stats
For this reason, we should backport the user-facing changes so that people using older versions can update their config files and CI scripts to use the new terminology. Without this, people may be forced to use the old, offensive terminology for years, just because they want to test their packages on PHP 7.1, for example.
PHPUnit 7 is no longer maintained and PHPUnit 8 receives bug fixes only.
I think we have to make an exception for this, if you are serious about removing racist terminology from the codebase. One has to acknowledge PHPUnit 9 has low adoption (~10% of monthly installs in June!), and so this needs to be fixed in more versions. Only changing in PHPUnit 9 is effectively not doing anything for (more than) 90% of people who are forced to use the old config file formats.
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I think we have to make an exception for this, if you are serious about removing racist terminology from the codebase. One has to acknowledge PHPUnit 9 has low adoption (~10% of monthly installs in June!), and so this needs to be fixed in more versions. Only changing in PHPUnit 9 is effectively not doing anything for (more than) 90% of people who are forced to use the old config file formats.