The publisher is referring to Metal Gear Solid 2 and 3 (plus any anthologies containing those games), which Konami de-listed in November because its license to historical footage used in the games’ cinematics had expired.
It is the progenitor of the nine-game Metal Gear and Metal Gear Solid series, whose most recent entry was 2015’s Metal Gear Solid 5: The Phantom Pain.
Konami has since published only one Metal Gear title, 2018’s zombie shooter Metal Gear Survive, which was a critical and commercial bust.
Metal Gear, developed and published by Konami, launched July 13, 1987 in Japan for the MSX2 personal computer.