Something I realised looking at Google+ identifiers -- 21 digit numerics, 19 of those significant -- was that it made brute-searching the user profile space infeasible. There were only 4 billion and change legitimate profiles, there was a 4 in 100 billion chance of hitting one by chance on any given random request of the space. And IDs appeared to berandomly distributed.

And yes, Google also posted a sitemaps file (or rather, 50,000 sitemap files) with all profile IDs. But that was last marked updated in March 2017, for some reason. Being able to validate that would have been nice.

But as a mitigation against blind bulk scrapes, a useful tool. I'd consider that one of G+'s good design elements.