Managing backward compatibility
Yes, this has been addressed by the core maintainers. You are safe if your module doesn't use any APIs deprecated until Drupal 8.8/8.9:

Drupal 8.8 was the final release to introduce new deprecations that will be removed in Drupal 9

A quote from the Drupal release team: https://groups.drupal.org/node/535670
You should wait to remove deprecations introduced in Drupal 9 until you prepare for Drupal 10 and don't need Drupal 8 support any longer.
So beginning with Drupal 8 you can manage compatibility for two Drupal versions  at the same time.