Ah yes I was about to update my answer. You can still download the latest installer from the t-splines website: http://www.tsplines.com/resources/releases/4/TSplines4.0_r11183ForRhino.exe
I'm also updating the links in the videos to this link. It's a shame really... But I guess its just how it is. All the best and good luck
Hi Jon, Thanks for noticing this. I'm not sure if you aware, but Autodesk bought T-splines a couple of years ago. They said back then they would keep it running fir Rhino... Unfortunately that promise went out he window and they're killing it. They're integrating it with there own software called Fusion. Read more here: http://www.tsplines.com/blog/2016/11/autodesk-plugins-for-rhino/
I would advise you to search the internet for the T-spline installer, i bet you can find something =)
Best of luck and a sad end of T-splines for Rhino
Arie