The first feature release of Drupal 9 includes the new experimental Olivero frontend theme and various additions to the Claro administration theme. Installer performance is improved 20% and full Composer 2 and PHP 8 support is available. Images with known dimensions are set to lazy-load by default to improve frontend performance.
Source: Drupal 9.1.0 is available
Looks like PHP 8, Composer 2, Symphony 4,5,6 will get us to Drupal 9. Upgrades paths from Drupal 7 are stable.