In hopes of finally addressing ongoing network-related issues in OS X Yosemite, the new 10.10.4 beta release does away with discoveryd and rolls back to mDNSResponder from OS X Mavericks. Apple replaced mDNSResponder in OS X Yosemite with discoveryd for handling certain network tasks, but that decision left many Mac owners frustrated with heavy CPU usage, slow Internet issues, and network connection problems.
Apple Kills Yosemite Network Bugs with Mavericks Code
Network-related issues cropped up for many Mac users as soon as they installed OS X Yosemite 10.10, and have persisted through every update so far. Rolling back to mDNSResponder most likely means Apple hasn't been able to find a way to resolve the issues introduced with discoveryd.
Since the OS X 10.10.4 update is still beta and not an official release, Apple could reverse course and bring back discoveryd before rolling it out to the general public. Considering the issues Apple has had so far it's likely that if those problems aren't resolved, however, discoveryd will stay on the sidelines for the foreseeable future.
[Thanks to Ars Technica for the heads up]