However, it is of a form that it can only be applied to new questions. We have many old questions with answers that do not satisfy the policy, what should we do about them?
I propose that we edit the resource recommendation banner into these questions, making the questions follow the policy.
As for the answers, those which do not follow the policy can be collected into a single Community Wiki answer. Maybe with a disclaimer about how it is no longer considered a good answer to a recommendation question. We could alternatively historical lock it, though the historical lock post notice seems to talk about questions even when applied to an answer.
This way, there aren't any broken windows, but the content is preserved. This requires some effort, though. I don't mind doing it myself, but I'll have to start a week from now. This isn't something urgent, so we can handle it at our leisure.
I rather like @Emilio's banner for the community wiki questions:
This answer contains some additional resources that may be useful. **Please note that answers which simply list resources but provide no details are strongly discouraged by [the site's policy on resource recommendation questions](https://physics.meta.stackexchange.com/questions/4697/are-resource-recommendations-allowed)**. This answer is left here to contain additional links that do not yet have commentary.
This answer contains some additional resources that may be useful. Please note that answers which simply list resources but provide no details are strongly discouraged by the site's policy on resource recommendation questions. This answer is left here to contain additional links that do not yet have commentary.
Move the content into the relevant tag wikis, then delete the old questions. Those who have volunteered to be custodians of book questions (or at least, that subset who aren't currently suspended) should do this tag-wiki curation before any new book questions are started.
That way, no useful content is lost, and we find out if there's genuine maintenance commitment or not.
(Kept this answer here for voting, I don't agree with this myself)
An alternate idea is to simply (permanently) historical lock all the old questions. This has the disadvantage of disallowing new answers1, however it is less effort, especially as many of these are already locked.
1. This can be partially remedied by allowing users to repost the question conforming to the new policy. That causes some unnecessary duplication of content though.