Problem/Motivation
We deprecated most jQuery UI library definitions, and removed these from Drupal 9.x
Some library definitions were not deprecated though, because they're dependencies of core JavaScript - drupal.dialog, drupal.autocomplete etc. which have not been refactored yet. This includes jquery.ui.button jquery.ui.mouse jquery.ui.resizable jquery.ui.widget, and jquery.ui itself.
However, those core libraries like drupal.autocomplete don't have to depend on jQuery UI libraries specified in core.libraries.yml, they could instead depend directly on the components themselves, essentially merging the library definitions together.
The big advantage of this is it allows us to avoid adding to the skipped deprecation list, see #3098489: Remove deprecated jQuery UI library definitions which is where this idea developed.
This doesn't reduce our dependency on the components themselves, but it prevents new dependencies on them being added in contributed and custom modules during the 9.x cycle.
Proposed resolution
1. Where an actual core library like drupal.autocomplete depends on jQuery UI components, add the components as direct javascript and CSS file dependencies, removing the dependency on the library definition.
2. Deprecate all of the remaining jquery.ui library definitions in core, ensuring that contrib replacements exist (true for most but not all as I type this).
Remaining tasks
1. Should we do a late deprecation for 9.x removal in 8.8 and 8.9? An argument in favour is that jQuery UI is already abandoned, and we already made a big announcement about this, we're just adding more to the same change record. If not, we'd need to decide whether to add an early 10.x deprecation (since we actively want to discourage dependencies on jQuery UI) or wait until 9.1.x
2. Views UI uses jquery.ui.dialog directly, instead of drupal.dialog. This makes it harder to deprecate jquery.ui.dialog. One option is to have Views UI declare its own views.jquery.ui.dialog library that's a clone of the jquery.ui.dialog definition. Another is to rely on the skipped deprecation list for jquery.ui.dialog
User interface changes
None.
API changes
Yes, more jquery.ui library definitions deprecated.
Data model changes
Release notes snippet
The core jquery.ui, jquery.ui.autocomplete, jquery.ui.dialog, jquery.ui.draggable, jquery.ui.menu, jquery.ui.mouse, jquery.ui.position, jquery.ui.resizable, and jquery.ui.widget library definitions have been deprecated. Modules and themes with a dependency on these library definitions should use the contributed module versions.
Modules that override these library definitions, or the definitions of libraries that depend on them (such as drupal.autocomplete, drupal.dialog, and drupal.tabbingmanager) may need to update their overrides to ensure the same changes to the resulting JavaScript and CSS are applied.