Just my opinion but I believe it was always "to this day" , but some people confused it as 'til this day and that started to be used also. The word until is a pretext to something happening post "this day". For example, 'Til this day, not many people have shopped here. To this day, not many people have shopped here. They mean different things. The first one is proclaiming that starting today, people have started to shop here. The second simply states not many people shop here ever.