Yeah, the wayback machine is the best tool for downloading the old versions. I have also uploaded backup copies to my cloud service so they're there for me. Until UMMU and UCGO have been updated to 2016, or the alternatives being worked on by Woo and Loru are released, I am still mainlining Orbiter 2010 like many other Orbinauts.
While Orbiter 2016 is stunning and an excellent update, Orbiter 2010 remains the most accessible version for simulating fully-fledged missions.