DF YouTube (Distraction Free) v1.13.504 
Everyone should use uBlock Origin in combination with this add-on! I have no affiliation with uBlock Origin. It's just an awesome extension for blocking ads across the internet, including on YouTube™...yes, that means you can say goodbye to video ads forever!
NOT WORKING?
Please use the feedback button to alert me if YouTube™ is not being displayed properly. YouTube™ occasionally updates its styling which may break DF Tube. I also recently ported this to Firefox from Chrome so let me know if there are issues on FF.
REGARDING PRIVACY
I do not read, examine or track your browser history or any other personal information. The only permissions I ask for are "tabs" to insert the functionality and "storage" to store your settings. I am a strong believer in privacy. This extension is just a tool to help remove the distracting elements from YouTube™.
The permissions used by this extensions are 1) Storage - to store your preferences, 2) Tabs - to load the DF YouTube script on each YouTube™ tab you open, 3) Notifications - probably for compatibility with the Chrome version, and 4) https://api.mailgun.net/* domain - which is a service I use to send your feedback to my email.
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YouTube is directly incentivized to create as many views and ad clicks as possible. As a result we are inundated with tasty thumbnails and advertisements from every angle. While this is good for someone looking to kill time, it's not so good for those of us intending to focus on educational content, do research, or simply find some relaxing music to groove to.
After using DF YouTube for a few days, you realize the extent to which the pattern of click, watch, scan mindlessly for a new video, click, watch, and repeat, has been ingrained in us by YouTube's design. DF YouTube breaks that cycle.
By default, DF YouTube hides the video recommendations sidebar as well as the related videos that appear at the end of videos. It also removes the grid of recommended videos that appear on your homepage. These settings are all configurable. It can also hide comments and disable playlists sitewide.
I hope DF YouTube allows you to focus on the content *you* want to focus on. Give it a try and be sure to report any suggestions or bugs!
***UPDATES*** (11/18/2019)
- Fixed issue where feed shows for a moment before being hidden