Youtube - Hide Watched Videos by voitek

Hide watched Youtube videos automatically hides videos you've already watched on YouTube creator channel pages, so you can focus on content you haven't seen yet. It works by reading the watch progress bar that YouTube displays under each video thumbnail — no separate tracking or account access needed.

You can configure the hiding threshold (1–100%): set it to 90% to hide only videos you watched to the end, or to 1% to hide anything you've started. Hiding can be toggled on and off with a single click from the toolbar. The extension is available in English and Polish.

Github:
https://github.com/voitek123/hide-watched-youtube-videos