Haven't saved anything myself yet, but I figure it won't be much different than other DRM. Some extensions
(use Chrome) will allow you to save the video (it's still encrypted however, and won't play). From there you can use ffmpeg to decrypt the video using the key in the .mpd file which you can get using "The Stream Detector" addon.
There's probably an even easier way using streammate, aria2 or yt-dl with cookies enabled, I honestly just can't be bothered to check.
It's only a matter of time before someone makes it just as trivial as it was before, if they haven't already.
However, it's DRM, so the tools to explicitly do it likely won't be hosted on github or anywhere you'd expect to see them.