How `window.wp.oldEditor` is being set in the editor?

Ah … Got it. It’s not related to Gutenberg editor but WordPress core itself:

window.wp.editor is being initiated here with those methods:
https://github.com/WordPress/wordpress-develop/blob/9ed92faaca92f66ec9e2979c9d896ddab3e4e993/src/js/_enqueues/wp/editor/base.js

And then, window.wp.oldEditor = window.wp.editor:
https://github.com/WordPress/wordpress-develop/blob/5868743f1f2b05c93642c9eebf89cb7f6610d6df/src/wp-includes/script-loader.php#L369