Dependencies of jQuery UI are being deprecated in core (*). When this happens, it may affect any site that depends on the jQuery UI Bridge feature of Bootstrap theme.
jQuery UI Bridge in Bootstrap is broken with Drupal 9.2.0 out, due to changes in the way the libraries in core are defined.
Is it possible to provide a version of Bootstrap theme that directly depends on jQuery libraries that are provided by contrib projects https://www.drupal.org/project/jquery_ui →
(*) references:
Active
4.0
Miscellaneous
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
This theme will not be supported for Bootstrap 4. See alternative themes for this support.
It seems that you just closed the issue without even reading the context. Who even said something about Bootstrap 4?
The MR is set against the 4.x branch and not the 3.x branch.