Basically Google wanted to put checksums in webpages and then not render the page period if the checksum didn’t match and said checksum could only be verified by “approved” browsers that had the correct certificate (which surprise was Chromium only browsers such as Chrome and probably Edge). As such you wouldn’t have been able to run any adblockers as that would change the checksum and the way the page was rendered. They could also then go one step further and do a Denouvo type set up to make sure the OS wasn’t being altered.

Okay, so I originally was going to go in a long rant about how they're still doing it, but decided that it didn't really add much to the comment, so removed it.

Afaik they've, for now at least, shelved it in browsers, but are still going ahead in Android webviews (as part of their war on Youtube Vanced).