My idea was that the currently unsupported combination of NOT NULL and no DEFAULT would mean "has to be assigned to a non-NULL value before it can be read from, or an exception is thrown". Solves the most common use case and is backwards compatible.
That won't allow you to use a variable in multiple places though... is there a reason we couldn't support something like IS DEFINED and UNSET?
I don't understand what your use case is. Could you demonstrate that with some code you'd write if these features were in?
One use case is NEW and OLD in triggers. Checking to see if one or the other is set is easier than checking TG_OP. It's also going to be faster (probably MUCH faster; IIRC the comparison currently happens via SPI).
This sounds useless.
Another case is selecting into a record:
EXECUTE ... INTO rec; IF rec IS DEFINED THEN ELSE EXECUTE <something else> INTO rec; IF rec IS DEFINED THEN
And this a workaround for non-functional FOUND.
I can't get excited about this idea based on these examples.