I think if I took it too far and said that all contractions are basically acceptable, y'all'd'n't've agreed with me.

Isn't formality itself a bunch of arbitrary rules? There's rarely anything about any formality rule that makes the thing itself inherently more or less polite, the point is that choosing to follow those arbitrary rules communicates that you are (or aren't) choosing to be formal about the thing. It's like a giant tone marker for "respectfully"

Y’all’d’n’t’ve is one of my favorites

I consider the arbitrary rules that we call formal English to just be the set of rules that lead to the most widely understood texts, so if you want to reach a broad audience, both across the world and across time, then keeping to those formal rules makes sense.

This is the one that still ends up in my technical writing.