The new modules and Drupal themes, for example, Barrio theme has the latest stable version 5.5.2 or 5.1.5. Is the old 8.x-4.33 system no longer to be used? Has the versioning system been changed because the modules or themes will be compatible with different versions of drupal or is there another reason?
Drupal is transitioning to semantic versioning..
modules can be compatible with drupal 8 and 9.
read about it here