[$] Whither WireGuard?

It has been just over one full year since the WireGuard virtual private
network implementation was reviewed here.
WireGuard has advanced in a number of ways since that article was written;
it has gained many happy users, has been endorsed
by Linus Torvalds, and is now supported by tools like NetworkManager.
There is one notable thing that has not happened, though: WireGuard
has not yet been merged into the mainline kernel. After a period of
silence, WireGuard is back, and it would appear that the long process of
getting upstream is nearly done.