According to the study:

In release - For the purposes of this study, a game is considered to be “in release” if the game, or a version of the game derived from its original release including emulated, modified, or reimplemented versions of the original game is reasonably, readily, and legally available from the game's rightsholder, either in physical or digital format, for a currently produced or supported game platform.

They are expecting copyright holders of a game to indefinitely sell the game. It's not that easy. Something like the 1999 movie Dogma would not qualify as "in release" since you can't buy it or watch it from the copyright holder. You can however buy a DVD version on amazon from mediamaniasales legally.

Overall, the study is expecting studios to support and release classic games for current or supported game platforms. It's a lot of work and it's not reasonable to expect a studio to do. If people want supported classic games then they should create a system that gives money to people trying to do that. We live in a society that requires money and people work to get that money. Expecting people to put in that work for free is pretty silly.