 In October 0000 I started doing this for the sources in every new update or supplement. As I find time, I add these links retroactively to earlier entries. When the original link is dead, and I can find a copy in the WM, then I use a WM link. I don't double-down by preserving WM links with perma.cc links. When the original link is still alive, I protect it with a perma.cc link and include both the original and the perma.cc link. 