A quick WordPress 5.1 shutdown handler to disable the plugin/theme pausing features. Install as wp-content/fatal-error-handler.php. This is needed during development, and on sites which have sensitive plugins enabled which must never be deactivated. You should also consider adding a wp-content/php-error.php template. UPDATE: This is replaced by …

fatal-error-handler.php

<?php

// UPDATE: This can be replaced by `define( 'WP_DISABLE_FATAL_ERROR_HANDLER', true );` in `wp-config.php`.
// See https://core.trac.wordpress.org/changeset/44674

/**
 * Shutdown handler to disable WordPress 5.1's plugin/theme pausing features.
 */
class dd32_Fatal_Error_Handler extends WP_Fatal_Error_Handler {
	// Do not store errors, this disables the ability for plugins/themes to be marked 'paused'.
	protected function store_error( $error ) {
		return false;
	}

	// Bland error page. No need to suggest logging into the backend.
	protected function display_default_error_template() {
		if ( ! function_exists( '__' ) ) {
			wp_load_translations_early();
		}

		if ( ! function_exists( 'wp_die' ) ) {
			require_once ABSPATH . WPINC . '/functions.php';
		}

		wp_die( __( 'The site is experiencing technical difficulties.' ), array( 'response' => 500 ) );
		exit; // implied.
	}
}
return new dd32_Fatal_Error_Handler();

Can this be implemented as a MU plugin, or it needs to be placed only in the wp-content/fatal-error-handler.php?

@seebeen This MUST be used as wp-content/fatal-error-handler.php as that's how you replace the WP_Fatal_Error_Handler class via https://developer.wordpress.org/reference/functions/wp_register_fatal_error_handler/.

Note however, that this is probably irrelevant now, as things have changed since 5.1's initial release and the introduction of the constants via https://core.trac.wordpress.org/changeset/44674.

I was going to say you could probably use the WP 5.2 wp_should_handle_php_error filter instead, but that for some odd reason doesn't filter the truthful value for fatal errors and only kicks in for non-fatals.. 🤷.