RedditMute από SmallUsefulProjects

This extension works for both new and old reddit.

You can block by pressing the "Block" button that appears below a post which will block the whole subreddit. Or you can open up the extension an manually enter subreddits.

The extension accepts multiple formats for blocking subreddits example:
https://reddit.com/r/example OR example OR /r/example. It also accepts comma separated lists.

There is also the functionality to block posts with keywords.

Finally the extension have two quick blocks that is targeting politic related keywords and subreddits.