And even if they somehow weren’t, we’d just do what we used to do with documents to turn them into "chewable bites": chunking, extracting, summarising,…
What I needed to do was sit with a user (not a manager/the person buying my services) and ask them to show me the different things they did with the software. Then I could write a spec for the actual _feature_ and would only need to look at the existing codebase if they needed data transferring across[1]. I don't see why our new LLM-based future would be any different
[1] Of course this meant I would leave out edge-cases and/or weird quirks of the system - often this was actually a bonus as they were either no longer relevant or worked that way because that was the only way they knew how to do it