Major League Baseball’s replay review system was implemented in 2014, after high definition cameras helped fans watching on television see more and more blown plays. It’s generally done well, though occasionally reviews take too long (my suggestion: limit them to two minutes, if the review crew can’t figure it out by then, it’s “call stands”).
One thing that we have NOT had in the six seasons of review is an explanation in the ballparks (and on broadcasts) on microphones via the PA system, as they do in the NFL and NHL.
In 2020, we’re going to have exactly that:
New for the 2020 MLB season: Much like NFL referees have done for years, umpires will be mic’d up and tell fans in attendance and those watching on television and listening on radio if reviewed calls are upheld or overturned.