One of the worst things you can do for preparing for PHP upgrades is to use ~E_DEPRECATED in your development environments. Turning off and ignoring deprecation messages causes a world of pain when upgrading.
Hard agree. But sometimes it's necessary to shut up noisy plugins (or core on PHP 8.1+) just while working on a problem, so I have a gist for that.
Turn on WordPress' WP_DEBUG_LOG but without E_STRICT. Accompanying blog post: http://snippets.webaware.com.au/snippets/wordpress-wp_debug_log-without-e_strict/ - debug-log-not-strict.php
@awoodsnet @ramsey in my past, that plugin has been WooCommerce, Easy Digital Downloads, Gravity Forms, every SEO and events plugin, you name it. Bumping your PHP version brings all the old code into the light.
Most of those are much better these days, but currently the WordPress core fills the log on 8.1, and you should see what 8.2 does!