And since you won't be able to modify web pages, it will also mean the end of customization, either for looks (ie. DarkReader, Stylus), conveniance (ie. Tampermonkey) or accessibility.
It doesn't seem to be targeting ad-blockers in particular (or other page customizing extensions), although that may result eventually. What it does do is let webpages restrict what web browsers and operating systems you are allowed to use, just like how SafetyNet on Android lets apps restrict you to using an OS signed by Google. That could end up with web pages forcing you to use a web browser and OS the big players like Google, Microsoft and Apple, blocking any less restrictive or less used competors like Firefox and Linux, thus creating a cryptographically enforced oligopoly. And even if they signed e.g. Firefox, it would only be certain builds of it. That would make it impossible to make a truly open-source browser that can access pages using this API. Quite concerning.
Users like visiting websites that are expensive to create and maintain, but they often want or need to do it without paying directly. These websites fund themselves with ads, but the advertisers can only afford to pay for humans to see the ads, rather than robots. This creates a need for human users to prove to websites that they're human, sometimes through tasks like challenges or logins.
Unfortunately what's going to happen in reality is that any non-standard ad consumption (including non consumption) will be flagged as fraudulent. "We cannot verify your activity, please disable your add-ons to continue".
This is straight up 3E for web browsers - it's a short road from this to forcing everyone onto apps and chromium, and good luck explaining to a politician why this is a big deal.
This year is going to show up in a lot of history books... Assuming we still make history books when all this is over