Hey Marius, while I understand that we need to sort the sites that have the error on them, one page on a client site had 26 items coming up on some page loads in admin – so we had to do something about it.
For others struggling with this.
In the plugin, you can edit the code. Note that when the plugin updates, you will need to do this again.
In the class-jquery-migrate-helper.php file go down to
HTML for jQuery Migrate deprecated notices
Comment out the following
<div class=”notice notice-error is-dismissible jquery-migrate-deprecation-notice hidden”>
This is a hacky hack as it leaves the function running, but it suppresses the display of the warning.
I’m sure our techs will find a better way to do this than the HTML type hack I’ve implemented!
This is mainly happening for an old page builder that’s still part of a live site, but that will be gone by the time WP 5.6 comes around.