Create 2-layered dropdown menus for custom taxonomy and custom post type

With a combination of jQuery / JavaScript and WP AJAX; you could look for an onchange event on the select box, send an ajax request to your admin-ajax.php with the year and then do a custom WP Query to return the html for the secondary drop down and append it to your dom.

https://codex.wordpress.org/Plugin_API/Action_Reference/wp_ajax_%28action%29

http://api.jquery.com/jquery.ajax

Once it is appending you can target your new select box with an onchange event as well.

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