What happened to the specific skeletons (the UE and SL rigs for example)?
The theory is that the “game engine” rig should be a functional generic replacement.
Outside that, assets for specific third part software has been broken out of MH and deferred to the online repos found here: http://www.makehumancommunity.org/content/user_contributed_assets.html (the rigs section is at the bottom).
As it is impossible for the MH dev group to support specific third part commercial software we don’t use or know ourselves, we have deferred such to user contributions areas. If someone feels they have the specific knowledge required to understand how rigs for a specific third part piece of software work, they are welcome to upload and maintain the rigs in those areas. At the time of writing this, Second Life / OpenSim have gotten such user support.
It is also possible you can find someone who still has the old somewhat shaky attempts at specific rigs lying around (we don’t). These are (obviously) open source and if someone wants to update or maintain them, they can upload the rigs to the user repos.