I don't think it is exactly true that recommendations are generally off-topic... For example, software recommendations are on-topic on Ask Ubuntu, although hardware recommendations are off-topic. The latter seems to be a relatively recent development too - the consensus about it developed over time, because, while most well-formulated software recommendation requests have continued to be answerable, reasonably durable, and not too broad (a handful of answers at most), hardware recommendations necessarily quickly become obsolete, and are very much opinion based, with little possibility for corroboration (until software recommendations - in the free software ecosystem, I can install, test and remove an application at no cost and with little effort).
All this is to say that, if the questions work, if they attract good answers, votes and views and thus help a large number of users, then they should be allowed I think.
The question you mention, to my thinking, falls into the too-broad (too many possible answers) and primarily-opinion-based (I like this book, you like that one) categories. But maybe I was too hasty in voting to close it - perhaps there is a small enough set of books specifically about the ethics of veganism to make that question manageable, and helpful to our visitors. I am not sure now!
Since the (soft|hard)ware recommendation sites are always up against the problem of questions being POB and too broad (and thus inviting bad answers), they have some nice sharp guidelines for what makes a good recommendation - software recs has these collected in a meta post which is linked in their help center.
I think we could perhaps aim for a nuanced approach along those lines - let's keep open minds for now and see what kinds of recommendation questions we are getting, and decide whether or not we want them, and if we do want some of them, figure out how we can make sure they are of good quality (ie answerable and useful to others).