How to stop WordPress from using utf8mb4_unicode_ci collation

You can filter every database query with the hook query, for example like this:

add_filter( 'query', function ( $query ) {
    return str_replace( 'utf8mb4_unicode_ci', 'utf8_unicode_ci', $query );
});

However, I would strongly recommend not to do this. Update your databases instead. The old utf8_* collations have very annoying limitations, and you will run into plugin compatibility problems with them.

And whatever you do, don’t use the general variations. They cannot sort properly.