Published notebooks can be imported by specifying their slug, but unpublished notebooks must be imported through their ID.
This distinction is not always obvious to new users. When they cannot import a link-shared notebook via its “d/”-prefixed path, they’re sometimes led to believe that they need to fork and publish the notebook in order to import it elsewhere.
Allowing import URLs of the form “d/{notebookId}” would remove that roadblock for new users.
Thanks for bringing this up. I have stumbled a few times on the inclusion or exclusion of /d/ working with different utilities (like the Handy Embed Code Generator). Since my usual approach is trial and error, making mistakes and trying variants comes naturally, but it’s also nice when tools are written ‘flexibly’ so that either w/ or w/o the prefix is acceptable.