In order to properly migrate to jQuery 3.x at least the following libraries need to be upgraded:

• malihu/malihu-custom-scrollbar-plugin (visible difference between jQuery 2.x and 3.x)
• grimmlink/qtip2 (jQuery migration plugin warnings)
• twitter/typeahead.js (jQuery migration plugin warnings)
• twitter/bootstrap (jQuery migration plugin warnings)
• etdsolutions/waypoints (jQuery migration plugin warnings)
• vakata/jstree (jQuery migration plugin warnings)
• components/jqueryui (jQuery migration plugin warnings)

Some of these "only" use deprecated features that are not per se dangerous and jQuery Migrate plugin (which restores old API) can be used in production for a while. See https://github.com/jquery/jquery-migrate/blob/main/warnings.md for more information.

Additional info: https://jquery.com/upgrade-guide/3.0/ and https://jquery.com/upgrade-guide/3.5/