
WordPress core, plugins, and themes can emit notices and deprecation messages on newer PHP versions. See the WordPress PHP compatibility guide for the current core support status.
The same is related to many popular plugins such as WooCommerce, Jetpack, Gravity Forms, and others. Mainly, they produce deprecation notices from the Action Scheduler library.
kagg-compatibility-error-handler.php into the /wp-content/mu-plugins/ folder. It contains the error handler, which loads earlier than any plugin or theme.