Closing this. It seems to now be possible to record on demand as well as save past 10 seconds into a clip with Steam+A and STEAM+DPAD up. The clip length isn't adjustable but that's a small and separate issue.
The followup issue about clip length is here https://github.com/ValveSoftware/steam-for-linux/issues/11434
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Please describe your issue in as much detail as possible:
Similar to Nvidia Shadowplay there should be an option and a hotkey for setting aside past 30 seconds for later editing. If you do longer game sessions or if your "Background recording disk space" is set low, you might want to set some highlight aside into permanent storage that doesn't get automatically overridden. The duration of the permanent recording should also be adjustable.