There's still progress being made, but WireGuard does do a lot of things to/for the in-kernel cryptography stuff. That's been the major holdup for getting it in. There's some that don't like that a new api is being made at all but I believe most are fine with that as long it shares as much internally as possible with the existing code so that there's not two copies of everything. A lot of that has been done last I heard but it wasn't quite ready yet for mainlining in that fashion. I suspect we'll see another RFC for it in the 5.4 merge window and maybe finally some traction at getting it into staging or all the way in.
EDIT: see sibling comment about wireguard and 5.4, apparently not slated for it this time but maybe 5.5.