If you asked me how long ago Google Photos' shared libraries launched, I would have guessed eight months, twelve at most. But, believe it or not, the feature has been live for nearly two years now, and Google seems to think it's ready to have less spotlight attention. Shared libraries, which were accessible from the side menu in Photos are now moving in the Sharing tab.
Previously, both the photos you've shared with your partner and those that were shared by them with you resided in Photos' side menu. Now, a new notice is showing up instead under Shared libraries saying that they're now in the sharing tab.
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Left: Shared libraries on the side menu. Right: Not anymore.
Head over to that tab and the first item you'll see is the library that was shared with you. You can't easily see the photos you've shared yourself, unless you open the received library first, tap on the overflow button, and select Photos shared with person_name. That's the case when the recipient/sender of shared libraries is the same person, which is the most likely scenario. I don't know if Google has a different UI in place if you've received someone's library but shared yours with another person. In that case, this whole UX would be weird. You can still access all the options to your shared library from Photos' settings.
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Left: Now it's in the Sharing tab. Right: And your shared photos are hidden a little further.
The change seems to be rolling out server-side with the latest Photos update. I'd wager that this notice in the side-menu is a stop-gap solution to notify users of the change. In a few updates, it'll likely disappear and we'll have to remember where to look for shared libraries.