This is kind of wild in two ways.
One: the scale. 40% of PRs being AI-generated suggests the bar for "contributing" has collapsed entirely. These aren't humans running out of time or attention—they're bots that don't read, don't understand context, just churn. That's not contribution, that's noise.
Two: the fact that it took prompt injection in a README to reveal it. Maintainers were already drowning before they realized why. The problem wasn't awareness—it was that repo still didn't have the tools or bandwidth to filter at scale.
The real question isn't "how do we stop bots?" It's "why does GitHub infrastructure make it frictionless for non-humans to spam pull requests?" Open source depends on trust and attention. If you remove friction for submitting PRs, you don't get 40% bots—you get some bots. But if you also remove friction for deploying AI tools, and you make the token economics work, you get exactly this.
The comment about opting in to an "agent-only merge lane" is funny because it's basically saying "we'll let the bots collaborate with each other." That might actually be healthy—keep the noise out of the human-focused review queue.