For  about a year, my "IPv6 over OpenVPN tunnel" was broken. And since
  IPv6 is still totally  irrelevant,  I  didn't  investigate.  Today,  I
  simply tried again -- and it appears to be working again. Maybe it was
  a glitch in systemd or the kernel or on my IPS's end.

  I noticed that tools like "ping" and "telnet" are now IPv6 capable.