Kyle Glaser at Baseball America had a look at the success rates of MLB teams picking prospects at the trade deadline. In the article, Glaser tracked prospects who went on to have a consequential MLB career. This means they played the majority of at least two seasons in the majors, with a positive career B-Ref WAR.
Glaser noted that the success rate of picking prospects went up from 2010-2017. From 2001-09, about one in six prospects added in deadline deals went on to be Major Leaguers. That improved to one in four, or 25.5%, from 2010-17.