I noticed this when looking at Jean Fiornoy's site in regard to the Dodal. which was, from memory, early 18th century. I have not done an exhaustive comparison but other decks like the Conver and the Fournier don't show Temperance like this. Other cards like le Diable and le Monde do appear to be bare breasted or close to it, however.
In general comments on Angels, the author of Meditations on the Tarot says that Guardian Angels are the manifestations of high and pure maternal love. Temperance is definitely a female angel, even when fully garbed and maybe the bare breasts were simply to signify that this is a female angel, and that she is, as others have said, caring and nurturing.
Maybe it was more common in earlier centuries to represent these characteristics of women in this way?