Linking to a specific timestamp of a YouTube video is not a new feature: you only need to append #t=1m30s or #t=90s to a YouTube URL to skip the first 89 seconds of the video (here's an example).
Now you no longer have to manually add the special parameter to the URL. Just right-click on the video and select "Copy video URL at current time". It's probably a good idea to pause the video before using this feature.
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