I don't believe non-shortest forms are common at all, except in deliberate variants like the Java version discussed earlier. That since it isn't easy to make an encoder that produces non-shortest forms through sheer sloppiness.
/.../ In any event there should be a #pike-goo to prevent old application from suddenly start throwing exceptions on previously accepted data.
I don't think that's a good idea since I consider this a security related fix that potentially stops exploits against old code. That's also the reason why I've patched all pikes back to 7.4 to not accept non-shortest forms.
What perhaps could be added is compat goo for the Java special NUL encoding, to cope with that specific case. It's not a common delimiter in ASCII-based protocols. Still, it could conceivably be used as an exploit for C-based libraries, although I'd like to think that Pike isn't very susceptible to that.