doing something like this would be more complicated than adding a field to the category. When saved the value would be saved to the category and not indicated what user is following.
This is not something that can’t really be done with ACF. You need to record information based on the user, probably in user meta, that stores a list of categories they are following and then query based on the value stored for the current user.
You could potentially use an ACF Taxonomy field on the user admin page and then use AJAX to store values to the current users page.
