Turns out that if they're unusable by LLMs they're likely unusable by human devs. If you follow sane clean coding principles (like not having godclasses) it turns out coding agents (and humans!) can understand and navigate your codebase, especially if you use recognizable patterns, even with very light documentation.
We judge long-term quality of human codebases (at least OS) by ongoing activity; for LLM codebases maybe a consistent or increasing level of activity is a bad smell?