It's sven coop actually. It's a free version of half-life on steam. https://store.steampowered.com/app/225840/Sven_Coop/

Whenever you try to join servers, use the steam server browser before so it detects which game its on. Yeah it's a little confusing, but gametracker doesn't support sven-coop. But it's basically half-life, so why not list it as such? Misleading, a little bit, but still.