It works well, but was missing two important things:
- The autoplayed video should match the videos shown on the right side of the screen when watching a video.
- The video should only be hidden if a configurable percentage of the video has been watched
Hopefully you can add them in the future :)

---
update: the author has now added percentage toggle, and notified me that the other feature is handled server side. Thanks a lot to the author!

Działa, ale za każdym razem po włączeniu przeglądarki otwiera się nowy tab "set youtube id".

works for me

This plugin change way I use youtube, my subscriptions is now clear to see what I didnt watch, only missing part for me is lack of option to hide "scheduled videos" so I see them when they are available.

it's working, but it need half second to do that, and like what said it will not hide www.youtube.com/feed/history , To avoid this feature become useless in youtube, anyway great add-on

Doesn't work at all

doesn't work on sidebar

At first, I thought this extension was great; it was doing exactly what I've been wanting from YouTube all this time: hide videos that I've already watched and that I'm done for, and also make space for other recommendations that I'd be interested in.

Unfortunately, the extension doesn't discriminate whether videos have been completely or partially watched. This means that it will also hide all partially watched videos from all possible pages on YouTube. This is a problem if you're in the middle of watching a long video and weren't able to finish it. Usually, these would (re)appear in your recommendations to remind you about finishing them; however, with the extension installed, all those videos will be hidden too and the only way you could keep track of them would be by bookmarking them and remembering they're there.

In addition, this extension creates all sorts of glitches all across YouTube, such as preventing playlists and the recommended videos rail to appear on video pages.

From what I can tell it works perfectly. I hope this keeps getting developed. It took a second to hide all the videos I watched on a YouTube channel but it did work.

Version 1.9
The blocking bugs associated with the videos tab, playlist tab, and playlist's page still exist but refreshing on any of these pages can get the videos loading properly. If I had to guess, YouTube loads a limited amount of videos at once (while also flagging the watched ones) and if enough videos get hidden at once (all of said flagged videos at the same time), it screws up the automatic loading of more videos.

Version 1.8
Works exactly as advertised! Almost too good. Being new, it still needs some filtering features. Currently, the History menu is useless (hilariously hides everything as it loads) unless the add-on is toggled off (thankfully one click with the toolbar icon).

Also, the ability to whitelist channels (such as subscriptions) would be great, otherwise toggling the add-on (like with history) works. Although, this does allow easy viewing of unwatched subscription videos when they remain visible (see below).

Playlists are also finicky. The setting for viewing your channel's playlists works perfectly fine as a whitelist. The playlists tab of other channels though can be blank if some videos have been blocked from the videos tab. This blocking can also sometimes cause the videos tab to remain blank and not load any other videos unlike the situation with the History menu (see above). The playlists themselves are untouched if you're watching them, even if they have watched videos in them. If you try to view all videos in the playlist from its page, then it will either hide viewed videos or hide everything if you don't refresh (see the easy viewing I mentioned above).

One last curiosity about the History tab, sometimes it will fail to load anything after it's blocked all of the loaded videos. But, if I scroll within the sidebar, more history items will begin loading and obviously blocked after a second (when the red view bar loads on them). Unfortunately, this scrolling does NOT initiate loading within the videos tab.

While already a functionally great add-on, there are some conveniences that could be added for better filter settings.

Hello, just tried your extension as a userscript (via Violentmonkey) and the subscriptions page in youtube doesn't work. It just shows today, yesterday, older etc as rows and everything is blank. However, the list view works.
Edit : It has been fixed.