That means it starts from Dragon Slayer: The Legend of Heroes (1989), and ends with The Legend of Heroes: Kuro no Kiseki (2021).
Falcom has shipped over 7 million copies of games in the Legend of Heroes series.
The latest entry in the Ys series is Ys IX: Monstrum Nox, which originally came out in 2019.
The exceptions to this are the Gagharv trilogy games, as The Legend of Heroes VI: Trails in the Sky (2004) introduced players to a new world and story.