Gareth, yes, I think sometimes that is right, if you have it linked to the user and it is so specific that the setting itself could help lead you to infer who the user was. So, it will depend on what the "personal" nature of the setting. Your theme example is a good one, or settings that are basic on/off or yes/no and are really generic.
But just making a choice per se is not identifiable: it would depend on the context. I chose tacos for lunch today but so did millions of others around me. When I lived in India and had tacos for lunch, I was the only one among millions to do that.
I've read interpretations of GDPR that subjective data can count as personally identifiable - if you have enough of them to be able to assist in re-identifying the person, which is easier if the choices are contextually specific and you have lots of them. For instance, for profiling.
Is whether someone has Moodle forum emails as a digest or one at a time
personally identifiable? I'd say no by itself. But tags from user
profiles? Definitely on the personal side of things because it would be easier to infer a user based on the uniqueness of the combination.
Assuming a normal not admin user, could someone guess it was your Moodle account if they had a collection of all of your user settings without the obvious personal items like location etc.?
That's an answerable question: someone could do the stats on the distribution of various user settings and probably come up with a table to show the relative uniqueness and frequency of combinations. It might be interesting to see such data.