If the duplicate release is being created just so that you can put it in a Release Collection, then I think Collections should be updated so that users can exclude mediums from the release.
Otherwise, I don’t see why we would duplicate a release because a code present in its cover art has expired.
Yes yes, now I also think it’s better to detach the optional medium, that not everybody will get, in a separate release, with a link.
We used to have a link (with the word option or may) when releases h ad only 1 medium.
I agree duplicate release is less good and changing collections to exclude mediums would be overkill and strange and complex and weird. 
A release-release “includes downloadcode for” (or similar) relationship seems to me like the ideal solution.
Making a distinction on whether the digital download is exactly identical to the physical one vs. additional bonus tracks seems weird to me.
I think it comes under artist intent (as well as practicality) - a few extra digital bonus tracks usually aren’t intended by artists to be their own ‘release’.
Whereas if you doubled the whole track listing for a digital download code they would probably say “what the heck” 
I don’t think there will be a technically consistent as well as practical answer for all this tbh 
I found the relationship I was thinking about:
Maybe we could undeprecate the next disc (with bonus attribute) relationship for this specific physical+download case?
But then, where do we put this optional bonus download?
Maybe more in another release group than in the same, when the content is different (when only bonus tracks).
In the same when it’s the same content, even if it also has a few bonus tracks.
I missed this, but @reosarevok specifically renamed the Japan-specific “Music Card” to the MusicBrainz-proprietary “Download Card” and rewrote the description of the medium type. Not sure how you can interpret this to not be resolved the way I said.
On the issue of whether we create a separate release or not, in that link, from @reosarevok:
…that wasn’t about the format, rather about whether there should be a second medium on the same release or a separate digital release. Which is also an unsolved issue…