With YouTube having launched a paid music streaming service a couple of months ago, it was probably just a question of when this might happen, but it still came as quite a shock for most of its users now: Streamus, the extremely popular music streaming add-on for Chrome, has been grounded by YouTube by revoking its API keys on July 17.
When the handy little extension was covered in an article on TechCrunch in January 2014 it was made widely popular over night, and it managed to gain a total of about 300,000 users since then. While being a very comfortable solution for users looking for a way to stream audio from YouTube without having to keep the page itself open, there always were certain disputable points regarding the terms of service (ToS), as it never displayed the originating video or any commercials connected to that, which is required by YouTube.
In a rather extensive e-mail communication and even a kind of collaboration, YouTube and Sean Anderson, the developer of the add-on, seemingly tried to find a solution, allowing the extension to stay alive in compliance with the ToS, probably avoiding problems for YouTube with the music labels they are serving. Interestingly it looks like YouTube even tried to offer Anderson a position inside its Music team, which he refused because of a feeling they were just trying to "solve a problem" that way. This was purportedly also the point at which communications got more and more unfriendly and he was left with less options and support to make Streamus compliant.
Despite several attempts of Anderson to fix Streamus e.g. by showing the video to the audio-stream, he never managed to get the required ads to show, which was at one point admittedly a problem on Google's side, the two parties never really found each other, which resulted in total deactivation of the extension last week. In the final showdown, Anderson was given a mere 9 hours to completely adapt his add-on, threatened by immediate deactivation, which could only be prolonged because a slight problem with YouTube's API was found, but finally this only delayed the inevitable.
Many users are still active on Streamus' subreddit, begging for an updated extension or a revival and Anderson is still looking for a way to bring it back, but as it looks like YouTube isn't interested in serving those 300'000 user's needs, there probably won't be a quick resolution.