Maybe I am wrong here, but
define( 'WP_DEBUG', true ); is still available and works in installs 5.2 and after.
According to WordPress docs: https://codex.wordpress.org/WP_DEBUG
Starting with WordPress version 2.5, setting WP_DEBUG to true also raises the error reporting level to E_ALL and activates warnings when deprecated functions or files are used; otherwise, WordPress sets the error reporting level to E_ALL ^ E_NOTICE ^ E_USER_NOTICE.
Edit: Here is how things play out in the back end. https://core.trac.wordpress.org/browser/tags/5.2/src/wp-includes/load.php#L321
There is nothing in there that says it is deprecated or being removed. And that error reporting level are set correctly.
I just tested it in my environment by setting WP_DEBUG to true inside of the wp-config.php file. And it gave the same The site is experiencing technical difficulties. Please check your site admin email inbox for instructions message, but also gave me the syntax error I put into my site to test and break it. I am running PHP 7.2