Not at the OS level, but I’ve built a chrome extension that does this.
I had a hard time focusing in grad school. I’d be watching YouTube lectures or reading a Reddit thread on a subject I was studying, and all of a sudden It was like I’d blink and be watching some comedy video or browsing r/funny.
Rather than simply blacklist sites, The chrome extension I built uses a topic model to block webpages. You input the topics you need to focus on (for me it was often computer science and math), and the extension will block webpages, YouTube vids and Reddit threads unrelated to those topics.
You can check it out at ( https://GetOnTopic.com )
Perhaps "kiosk mode" software so you can only run certain app(s)?