I've had this problem since October 2018 on my Mac caused by a malware downloaded with a fake Java update. Malware was neutralized but this problem remained. Nobody could help, the only advice everybody gave me was to wipe out my HD entirely, which I didn't want to do. Andrew's idea helped! I looked for a file "manifest.json" anywhere on my Mac, and I found a dozen text files with this name in application support/google. One by one I read them looking for anything referring to youtube. And I found the one with a switch True/false in it after the word "Enable". I opened the file to edit it, which I could, and changed "true" to "false". Restarted the computer, and it worked like magic. After one year and a half I can watch youtube videos again on my Mac. Thanks, Andrew Savoy.
So, I had this issue with a Windows 10 laptop at my work. I work for a school district so we do have Google Admin with restrictions on content for students, but not the staff.
The SYMPTOMS: Certain Youtube content blocked. Google user account would show 'Restricted mode off' in quick view (clicking on user icon), but when actually going into settings, 'Restricted mode on' would show and it was greyed out from being able to be changed. Did not matter which Windows User Account was used, or which Google account was used. Same Google user account on another Windows 10 PC would allow 'Restricted Mode' to be toggled in settings, so I know it had nothing to do with my Google Admin settings. This was also across all browsers, so I know it was not strictly a Chrome issue.
The SOLUTION: I looked at other solutions, like registry for Policies\Google with safe search, but that registry key was not present. Wiping the cookies and cache across browsers didn't help. Uninstall/reinstall of Chrome didn't help. I dug in and found a registry key "\HKLM\SOFTWARE\Google\Chrome\NativeMessagingHosts\com.microsoft.browsercore" that pointed to "C:\Program Files\Windows Security\BrowserCore\manifest.json". I could not modify or delete that key. I also could not modify or delete the "manifest.json" file. What I did do was copy the file from a WIndows 10 PC that YouTube worked on to a flash drive. Then use a Windows 10 boot disc to enter into the Windows Recovery Environment and use the CMD interface to rename the existing file and copy the file from the thumb drive. Reboot, and YouTube 'Restricted mode' was off in both the quick interface and the full settings interface.
PROBLEM SOLVED.
Take that Microsoft.