I took a quick look at PHP, and saw what the hash
function there doesn't appear to support it. Looking further, it seems it may be a while before we see wide use of that algorithm. PasswordMaker doesn't support SHA3, and BLAKE was in the top 5 for that.
Then there's the Java (Android and desktop) and Python versions (which I'm not a part of, so I'm not sure what they use to calculate the hashes). If you can find whoever handle those versions, maybe they can chime in on how easy it would be to add. Then there's the issue of having the Firefox version updated.
For the most part, the JS based versions will be easy to add this to (And we should add SHA3 as well), the other languages however...
While I can see removing it from new versions (at least the broken HMAC-SHA256 as the bug there is nuts), I'm sure if there's anyone still using it, it would be bad for them. My stand, if you have already support them, keep it, if not, don't worry about adding it.