This is the first release of Metatag that is compatible with Drupal 9, thanks to several wonderful people who spent time debugging issues and rewriting parts of the module.
It should be noted that this release is compatible with Drupal 8.7.7+, 8.8, 8.9 and 9.0. There are some test regressions for 8.7 due to compatibility problems with the Page Manager and Redirect modules. It is no longer compatible with 8.7.6 or any release of 8.6.