To properly answer your question -

"Wondering if a proper 8 yard muted, ancient or hunting Kilt would be proper wear for formal/informal events"

The answer is YES!!!. There are no kilt kops, there are no written down, set in stone, rules about this. There really is no right or wrong.

There are some things that don't make much sense or which we usually do or don't do out of habit or common sense but not because there is some rule about it.

And if my answer so far has not convinced you consider this.

An ancient version of a Tartan and a Modern version of the same Tartan, ARE THE EXACT SAME TARTAN! The official thread count is all that matters. In a thread count if there is Green in the Tartan we use the letter "G". We don't specify what color Green. Just Green. All the versions are fully correct as long as you have Green.