Google Photos 1.20 has started rolling out, bringing with it the second social feature for the platform: comments in shared albums and photos, plus smart suggestions for relevant additions to a shared album.
Commenting is for either individual photos or an entire album. Cody already found evidence of it in his Photos v1.16 teardown, so it's not a total surprise it's turned up now. When enabled through an album's sharing options (the feature is off by default), there will be a button on the footer when viewing a photo, or "add a comment" is added at the bottom of an album. There's comment deletion, as you would expect, and a 'copy text' action too. Comments can also be reported for abuse. Like Cody expected, there is currently no editing - you either delete or leave it, there's no middle ground. When someone else comments on a photo or album, the owner will get a notification.
Left: an album with a comment. Right: an individual photo with a comment.
Regarding smart suggestions, if you are viewing someone else's shared album and Google thinks you could add a relevant photo, it will tell you this. I personally couldn't get it working, though; it's probably one of those features which will pop up only occasionally.
WHAT'S NEW• Comment on photos in shared albums• Add your photos to a received shared album in one tap with smart suggestions• Device folder performance improvements
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.