"major architectural decisions don't get documented anywhere" "commit messages give no "why""

This is so far outside of common industry practices that I don't think your sentiment generalizes. Or perhaps your expectation of what should go in a single commit message is different from the rest of us...

LLMs, especially those with reasoning chains, are notoriously bad at explaining their thought process. This isn't vibes, it is empiricism: https://arxiv.org/abs/2305.04388

If you are genuinely working somewhere where the people around you are worse than LLMs at explaining and documenting their thought process, I would looking elsewhere. Can't imagine that is good for one's own development (or sanity).

I've worked everywhere from small startups to megacorps. The megacorps certainly do better with things like initial design documents that startups often skip entirely, but even then they're often largely out-of-date because nobody updates them. I can guarantee you that I am talking about common industry practices in consumer-facing apps.

I'm not really interested in what some academic paper has to say -- I use LLM's daily and see first-hand the quality of the documentation and explanations they produce.

I don't think there's any question that, as a general rule, LLM's do a much better job documenting what they're doing, and making it easy for people to read their code, with copious comments explaining what the code is doing and why. Engineers, on the other hand, have lots of competing priorities -- even when they want to document more, the thing needs to be shipped yesterday.