It’s not missing from the site but listed as a game and not an app and doesn’t show on the apps list because of this. This is a piece of software for making most other games that don’t support borderless windowed mode run in borderless windowed mode.
Borderless Gaming
Pocketguns2's achievements in Borderless Gaminghttps://store.steampowered.com/app/388080/Borderless_Gaming/