Did lazy loading made it in 5.4?

wp-includes/class-wp-metadata-lazyloader.php is included in the list of revised files here:

Version 5.4

Full list of tickets included in 5.4. here:
https://core.trac.www.ads-software.com/milestone/5.4

This feature wasnt included into 5.4.
Most likely it will hit WP 5.5.

If you want to use native lazy loading (without JS fallbacks, which sometimes can break some images loading) you can try a feature plugin:

https://www.ads-software.com/plugins/wp-lazy-loading/

Its same code that will be included into future versions of WordPress