Deprecated core themes cannot be disabled on the "Appearance" tab because they are hidden

Problem/Motivation

After upgrading a website to version 9.5.0, the following warning appeared in the site's Status report:

My site is using Olivero as its default theme and Claro as the administration theme. Neither depends on Classy or Stable. I am unable to find a means to disable these unused deprecated themes. They do not appear on the list of themes on the "Appearance" page, where one normally uninstall unused themes. Screenshot:

There is a documentation page about Deprecated and obsolete modules and themes, but it just say that they are deprecated and suggests that one should use Starterkit theme generator instead. It doesn't tell one how to disable them.

The problem only affects legacy websites. A brand new install of Drupal 9.5.0 with Olivero and Claro installed does not get this warning.

I suspect that the reason this happens is that Bootstrap5 version 2.0.5 (which used Classy as its "base theme", which in turn used Stable) in the past was installed as the default theme. However, Bootstrap5 is no longer installed now.

Steps to reproduce

To reproduce, install Drupal 9.5.0 and set Bootstrap5 version 2.0.5 as the default theme. Then uninstall Bootstrap5 and revert to Olivero as its default theme. You shall now get the warning about these deprecated themes being enabled.

Proposed resolution

Remaining tasks

• There should also be some other indication added to the themes page (visual + text) to view and uninstall themes which have the hidden: true attribute (see workaround in #3328016-09: Deprecated core themes cannot be disabled on the "Appearance" tab because they are hidden.