Hey, I'm starting a new topic to talk about this, because my thoughts on the subject aren't really germane to the thread in which they arose.
To put it another way, Fi (and Ni) often provides some of the first principles from which Ti reasons.
On the other hand, from a quadra progression perspective, it's pretty easy to see that a big difference between delta and beta is the observation of conventionally accepted ways of life. And my point in bringing up taboos is to say that there is nothing at all wrong with following conventionally accepted rules that you can't "prove" logically; everybody does that, naturally and necessarily.
Nevertheless, you see a very beta "in the abyss" moment, for instance, in the Oresteia where the accepted rules of life (don't kill your own kin) are broken, first in the social/citizen sphere by Agamemnon, and then (more importantly) in the domestic/personal sphere by Clytaemnestra and then Orestes. Then you start questioning the logical validity (Ti) of those accepted ways of life (an alpha phase), and then you go into this beta phase where abstracted and personified principles war against one another (Ni) until a new truth is arrived at (Gamma phase).
The point is, in the beta phase, you have to suspend the taboos, open them up to Ti questioning (based on Se: what will get us the desired outcome), and then change something in how you look at the world, so that can be reified into new traditions of "how we do things here".
So yeah, summary, I'm not saying that Fi valuers think in terms of taboos. I'm saying that taboos are a fundamental part of every society, and are one of the areas of reality that we all need Fi for, whether we value it or not. Taboos are good examples of the sort of judgments that we need Fi to make, of the necessity of reactions that come from an internal place that we cannot justify or derive.
Also: taboos in the sense of social etiquette is NTR. But taboos in the sense of "judgments about the quality of a thing that are not derived (explicitly/logically) from experience or other postulates" are related to Fi. Fi IS the voice that says "we don't do that because we don't do that." Ti is the voice that says "we don't do that because action y goes in category x and category x is undesirable." Maybe a subtle difference, and they can certainly feel equally arbitrary, but there's a difference nonetheless.