With new javascript frameworks like React and Vue, what is the state of jQuery in Drupal?
jQuery is still commonly used in most Drupal sites that are not decoupled. jQuery UI is used in many parts of the system for browser compatibility and accessibility. There is some work going on to update it to something more modern.
UI Dialog
A modal/dialog popup can be created by adding the "use-ajax" class to a link, plus dialog parameters. https://www.drupal.org/node/3031444
This dialog uses jQuery UI Dialog, and is used in many places in the admin interface.
You can implement a different library on the front end using different techniques, but this is still the "native" dialog widget in use.
This issue tracks the progress on replacing it: https://www.drupal.org/project/drupal/issues/2158943
Once
jQuery Once is a plugin written by the Drupal community to load scripts once, even if the page is updated with ajax. It was replaced with a library called once.
https://www.drupal.org/node/3158256
Other issues
[meta] Replace JQuery with vanilla Javascript in core
https://www.drupal.org/project/drupal/issues/3052002
[Plan] Remove jQuery UI components used by Drupal core and replace with a set of supported solutions
https://www.drupal.org/project/drupal/issues/3067261
Proposal to use Vue.js for building Drupal’s administrative UIs
https://www.drupal.org/project/ideas/issues/2913628
Re-evaluate use of Backbone.js in core
https://www.drupal.org/project/drupal/issues/3145958
Search for javascript modernization initiative issues
https://www.drupal.org/project/issues/search?issue_tags=Javascript%20Modernization%20Initiative
Replacing(?) jQuery UI resizable
https://www.drupal.org/project/drupal/issues/3201835