Steam has a new game recording tool, and it works on Steam Deck
Which games have integrated timeline markers by developers?

Valve has integrated timeline markers in Dota 2 and Counter-Strike 2, allowing developers to mark notable moments on the timeline during gameplay recording.
How can users activate manual recording in Steam?

To activate manual recording in Steam, users need to opt into the Steam beta branch, then head to the Steam menu, go to settings, and select either Steam Beta Update or Steam Families Beta. After restarting Steam, navigate to Settings, then go to Game Recording, and choose On Demand Recording to start recording gameplay footage manually.
What is the "Steam Timeline" used for?

The "Steam Timeline" is a feature introduced by Valve that allows users to capture and share gameplay moments easily5. It works by continuously recording gameplay or allowing users to manually record with a hotkey. The recorded clips can be shared through various options, including exporting as an MP4, sending to the Steam mobile app, or creating a temporary link. The Steam Timeline also lets users add markers to note interesting moments and allows developers to mark notable events in their games.