Must have gotten started somewhere before the SPLIT.
Oh yes, long before. The celebration of the Dormition of the Theotokos started in the 7th century at the latest, long before the Great Schism. Writings about her death go back as far as the 3rd or 4th century. St. John of Damascus, one of the Fathers of The Church, wrote three magnificent sermons on her dormition in the 7th or 8th century. Here's a link to his first one:
http://full-of-grace-and-truth.blogspot.com/2009/08/on-dormition-of-theotokos-by-st-john-of.html
The Holy Traditions surrounding the Dormition of the Most Holy Theotokos have been around for a very long time. It is only really within the last 175 years or so that there has been much controversy about it so far as I know.