I think such errors should be allowed. The rule about stderr output states that only fatal errors are not allowed. The important thing about error condition is whether it causes the program to terminate, not how it is called.
Strictly speaking, yash stderr output consists of errors: "cannot execute command" is clearly an error, yash (and other shells) just do not call it so.
It is a too weak distinction - how interpreter names an error condition, I think we should not rely on that.
For example, ><> interpreter prints "Something smells fishy" on all invalid commands - it also does not have t…