Enable jQuery Migrate Helper
by Marius L. J. on WordPress.org
With the update to WordPress 5.5, a migration tool known as jquery-migrate was no longer enabled by default. This may lead to lacking functionality or unexpected behavior in some themes or plugins that run older code.
This plugin serves as a temporary solution, enabling the migration script for your site to give your plugin and theme authors some more time to update, and test, their code.
With the update to WordPress 5.6, the included version of jQuery is also upgraded. This means that old code that previously caused warnings now may instead may cause errors or stop working entirely.
Some of the features no longer working will just stop working behind the scenes without any apparent problem.
The plugin will let you downgrade to a previous version of jQuery for a period, but as a site administrator you are encouraged to get the underlying issue fixed.
