jQuery UI is being phased out of Drupal core, and most libraries were deprecated in 8.8 and moved to contributed modules. jQuery UI Sortable has been replaced with Sortable.js, and jQuery UI position has been replaced with Popper.js.
The remaining libraries which could not be deprecated in 8.8.0 have been forked into Drupal 9 core to make them maintainable in case of any security issues before Drupal 9 end-of-life, with the goal of deprecating them all for complete removal prior to 10.0.0.
Drupal core previously included only the minified JavaScript files, now each remaining file has the matching source from jQuery UI 1.12.1.
Updating the forked JavaScript
Whenever the forked jQuery UI JavaScript files are modified (which should be rare), they should be re-minified. This can be done using yarn build:jqueryui.
