It's been way to long for this feature to not work. My guess it's a server side issue. Wireshark shows packet being sent up during a Check for Update check from the Help menu. What's in those packets I'm not sure. They very well could contain no information about the current version and the server has no idea what to send back. Simplify3D may have implemented this feature a different way, but which ever way it was done, doesn't actually work.
I've just tried the "Check for Update" in all versions from 3.1....ie 3.1, 3.1.1, 4.0.0, 4.0.1, and 4.1.....and I get a consistent "Simplify3D version 4.1.0 is now available...." message on all versions except 4.1, which I get the "Your software is up to date!" message.
I use Windows10. So if there's an issue, its not on Windows systems.