Just like when Google announced the changes coming to API deprecation and some APIs even being retired, Google is now looking at the Webmaster tools it can phase out. The company decides these tools' fate by seeing "if they're still useful in comparison to the maintenance and support they require."
The first tool to get the boot is the Subscriber stats feature. The reported "the number of subscribers to a site's RSS or Atom feeds." Google already has the same features included in their Feedburner tool so they suggest users of the current Subscriber stats feature switch to that.
The second removal is for the Create robots.txt tool. This allowed Web sites to generate a robots.txt file that would block a section of a Web site from being crawled by the Googlebot. The reason for removal is that it got very little use. Google says that those people who did use the feature can easily create their own since there are a multitude of other services that create robots.txt files.
The last feature hitting the cutting block is the Site performance feature that's part of the Webmaster Tools Lab. It let Webmasters check out the average load time of a site's pages. The reason for its removal is the same as the last - low usage. If you need to check your site's performance, Google provides the same features in the Site Speed feature in Google Analytics or Google's PageSpeed tool.
As you can see, these removals are more about removing redundancy than any kind of breaking changes. The retiring of these features only means that Webmasters have to switch to one of the many other options available. Chances are you're already using one of those alternative options. If you are still using one of the above features, you have two weeks to say your goodbyes.