Why is the module deprecated/obsolete?

I'm also wondering why this module is now obsolete?

No Drupal 9 version is available. The maintainer would have been contacted multiple times before this happened. The solution is now in #3249526: Offering to maintain CKEditor Advanced Tab.

It seems the project has been marked as obsolete because the project owner thought the feature has been implemented in core. For what I can see, Drupal core doesn't include the CKEditor plugin used from this module. May anybody confirm it's so?

As side note, once Drupal core uses CKEditor 5 by default, this module will need to be updated or it won't be anymore necessary.
Drupal core has now a new module for CKEditor 5. For what I understood, that module isn't yet ready to be used, and the old CKEditor is still installed and used on any Drupal installations using the standard installation profile.

I found CKEditor 5 is a new experimental module, which says:

CKEditor 4 was added to Drupal core in 8.x. It is a stable project but it will stop being supported sometime in 2023, around the same time as Drupal 9's EOL.

Replacing CKEditor 4 with the new and very much improved CKEditor 5 was identified as a Drupal 10 initiative.

I take the plan is to replace the CKEditor module with the CKEditor 5 module in Drupal 10. If this module needs to just be re-written for CKEditor 5, we can make a Drupal 10 branch, and keep the Drupal 9 branch for the sites that are using that Drupal version. In the case we still don't have a Drupal 10 branch when Drupal 9 reaches its EOF, we will mark this module as obsolete.

Do you think this is the plan we should follow?

I changed the project status to Maintenance fixes only.