The concept of sharing set groups of people for others to add was original and innovative, and launched alongside Google+ way back in 2011. Circle sharing was useful for sharing recommended people to add, and populating the stream with fresh, relevant posts. The feature was very handy in the early years, but diminished in use with the arrival of Communities in December 2012.
Unfortunately, the system had been abused heavily over the years, resulting in a lot of spam stemming from posts asking you to reshare to get included. Adding circles from posts that require you to add and share would mean junk in your stream, and little to no engagement from new followers. Give Top Contributor, +John Skeats' post a read to learn more about the good and dark side of circle sharing. It seems that the feature wasn't used legitimately by many people, which is a fair reason for Google to give it the axe.
The recently launched Google+ Collections is a good alternative for following and sharing content (rather than people) by topic, though sharing multiple at a time isn't possible, and for good reason.
Did you use the circle sharing feature?
Source: +CircleCount - Google+ post