[rjspotter]

That isn't an answer to the question.

Your response is selection bias. Every project that you've seen is a ball of technical debt. There is no argument to how documentation alleviates technical debt. If you document a poorly designed and scoped piece of software it doesn't make it any easier to change.

If the software was impossible to change and that change was a requirement then the code bases you were working with did not represent "working software"

The agile manifesto says, "working software over documentation." It does not say "working software and an absence of documentation."

The fact that you needed to add features is exactly the reason agile exists. If the creators had a complete feature plan in the beginning and faithfully executed it without deviation from the plan then there would be no need to ever add features and the quality of the code in terms of "changability" would be moot.