> [1] Said person said the appendices to C99 aren’t authoritative, but if something is in the spec, including in the appendices, it’s authoritative.

That's not true. There is a difference between normative text and informative text. Informative text is not authoritative, and you were citing an appendix that is labeled as informative.

> I am hoping the up and coming C2y spec has very clearly defined behavior when using unintialized memory (ideally where it will work but the bytes can have any values).

It won't. Uninitialized memory can't have "very clearly defined behavior" without breaking essentially every single implementation, and WG14 is very loth to break existing implementations.