Sorry, I mean the Heart column in the collection. If I remember the Help box correctly, they were called "Tracked Troops".
EDIT: I managed to recover the original text, and it says:
You can track and filter to your highest-priority troops by using the ♥ check boxes.
As I see, there are other ways of tracking all of what you said:
- Count column: >0
- Level column
- Rarity and Count columns
Except for traits, of course.
@Annaerith, what do you mean by completed mythics? Fully ascended, traits, both? For the ascension part, you could use the Rarity column, but the only reason I currently see for the Heart columns is to track traits. Am I wrong?