On 7 Feb 2019 11:07 -0700, from coff at minnie.tuhs.org (Grant Taylor via COFF):
> The only thing that comes to mind is IPsec's ESP(50) and AH(51) which—as I
> understand it—are filtered too frequently because they aren't ICMP(1),
> TCP(6), or UDP(17).  Too many firewalls interfere to the point that they are
> unreliable.

While different, I think that the introduction of the more advanced
IPv6 address formats into DNS also qualifies. I don't recall off hand
if bit-string labels (which were used for reverse lookups) or the A6
RRtype (for forward lookups) was the more problematic one, but I do
recall that both had issues that made real-world adoption non-trivial
in the best of cases.

-- 
Michael Kjörling • https://michael.kjorling.se • michael at kjorling.se
  “The most dangerous thought that you can have as a creative person
              is to think you know what you’re doing.” (Bret Victor)