When going through the list by platforms, it will also list games that got relisted, though some lack the [RELISTED] tag but are being sold as packages with other similar games (Doom 1+2, Heretic+Hexen), as remasters only (Soul Reaver 1 and 2, Metal Gear Solid 2) or as a "final edition" with all updates and DLC as if it was a different game (Super Lucky's Tale, Street Fighter 4)

For every online-only game, the reason was a matter of publishers no longer being able/willing to keep servers running - Super Monday Night Combat is a peculiar case because the reason for the shutdown was that their netcode didn't comply with European GDPR

For single player games, the most common case is copyright imbroglio. There is also the case of Tax Heaven 3000, which was delisted from Steam before release, but it remains available on itch.io

The lists also contain the shovelware asset flip clones, like "The Last Hope" which surprises nobody when they get delisted, really.