Complex sentence: D.C. United of Major League Soccer played at RFK Stadium from the team's debut in 1996 until 2017; at the time, RFK Stadium was the longest-used stadium since the league's inception.
Split into simpler sentences: D.C. United of Major League Soccer played at RFK Stadium from the team's debut in 1996 until 2017. At the time, RFK Stadium was the longest-used stadium in MLS since the league's inception.