cc: @astearns The key is to find a term that indicates that the value is about getting rid of orphans, but not in all circumstances, and not at any cost.
avoid as suggested here by @fantasai is already used in CSS (see https://drafts.csswg.org/indexes/) in a number of places with the meaning "don't, unless [X]", so I think it can work: the precise criteria for when not to do it is left up to the UA, but it is there.
To me, reduce could work too, but the meaning is almost too weak: instead of a rule generally applied with exceptions as implied by avoid, it would seem to indicate a slant, a preference. It's not wholly inappropriate here, especially given that everything is largely up to the user agent anyway, but that seems weaker than the author intent we're aiming for.

I’m OK going with avoid for now. But I do think that authors are going to expect that all orphans will be eliminated, and UAs absolutely should not do that.

authors are going to expect that all orphans will be eliminated

If they don't think about too much, probably. But if you show them examples of cases where it would be bad to do so, and it doesn't do so, I suspect they'll be happy with the result, since forcibly doing it would be ugly.

I’m OK going with avoid for now. But I do think that authors are going to expect that all orphans will be eliminated, and UAs absolutely should not do that.