The question is going to be: is it a USEFUL signal. I suspect not. And frankly, to be honest, as a senior developer who uses AI assistants routinely, I would consider it a serious disincentive to actually submit a PR in the first place to a repository that has such a policy. Submitting a good PR is hard work. Often an order of magnitude more work than the fix itself. If I think that a repository is going to not accept my PR just because I use a coding assistant, I'm going to be much less inclined to submit a PR at all.