Of course, and there's no reason to write such code.
I just dislike that the simple syntax rule "any expression can be used as a statement" now has an exception.
I haven't been able to think of scenarios where that might have consequences (code generation or refactoring tools?) but that doesn't say much as I'm not that smart.
Edit: having looked at the cases that are disallowed, they remind me of generator expressions. Those are usually written with parens, that can optionally be omitted in some cases. := is the same except they can be omitted in so many cases that it's easier to list the cases where they can't.
I think a generator expression used as a statement already requires the parens, even though they can be omitted e.g. as a single parameter of a function call. So that's probably ok then.