It’s not just the games that support it, it will also tag other metadata it knows automatically on all games, like achievement unlocks. And you can manually add tags, too. It’s all telemetry that Valve and the devs are already extracting from you, of course, plus whatever you volunteer in manual markers, but it’s still a bit creepy to see it laid out on a timeline like that.
I don’t think that’s true of either. They’re not saving all this video in RAM. Steam’s system even asks you during setup to choose a temp folder for the video to sit in until you choose to save it as a clip. I don’t know when it flushes that cache, but I’m assuming it doesn’t and simply overwrites it when it runs out of the allocated space to allow you to go back and save something after you close a game.