`"No Distractions" for YouTube` works in a very simple manner: it injects a style sheet to youtube.com which locally hides recommended videos.
After youo have installed this app, the extension will appear in Safari > Preferences... > Extensions . Here, you can turn the extension off and on - or uninstall it.
If you have any problems, contact me at ulrik.lyngs@gmail.com or create a GitHub issue on the repository github.com/ulyngs/open-source-no-distractions-for-youtube/.