I am not talking about editing an existing slug; I am talking about specifying a preferred slug before the record is written to the database.
WordPress does not work that way. The initial slug is produced from the title, and then checked against the existing slugs to see if there is a conflict, in which case a number is added to the end to prevent the conflict.
Basically, you don’t choose the initial slug even in the current system. This has nothing to do with the editor, it’s how WordPress works.
In the classic editor, I am able to specify a preferred slug (in the ‘slug’ field) before the database record is created.
Nope. Doesn’t work that way. In the current editor, you type in the title, then the Permalink field appears with the link made from the title. This means that in the background, the auto-draft saved. That slug is in the database. It may not be as obvious, but this is the exact same behavior.
There is no way to input a slug or edit the permalink before the post is saved to the database.