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Whether this was a conscious design choice from the team or not, I don't know, (and yes I should have posted this months ago), an unlisted synth is no longer notated in any way to be 'Unlisted' to those that the link is given to.
By contrast, if someone sends me a link to a private YouTube video, it shows me at the top of the video that it is private - thus providing me with a cue that I should probably not share it with anyone. Photosynth used to do this for unlisted synths, but no longer does so unless you are the author of the synth or pano and are signed into your account.
You can argue that a person sending a link to an unlisted synth would just verbally tell the receiving party not to share the link, but that's just shifting responsibility in my eyes.
Perhaps you could also argue that displaying the 'Unlisted' status could invoke a desire in the receiving party to share the video, similar to the quandary of whether telling one's friends|classmates|workmates about your injured foot will lead to more intentional strikes than would have accidentally occurred, but I'm thinking that this was more of just a technical oversight than a philosophical debate. ツ