This post title is misleading.

They aren't proposing a way for browsers to DRM page contents and prevent modifications from extensions. This proposal is for an API that allows for details of the browser environment to be shared and cryptographically verified. Think of it like how Android apps have a framework to check that a device is not rooted, except it will also tell you more details like what flavor of OS is being used.

Is it a pointless proposal that will hurt the open web more than it will help? Yes.

Could it be used to enforce DRM? Also, yes. A server could refuse to provide protected content to unverified browsers or browsers running under an environment they don't trust (e.g. Linux).

Does it aim to destroy extensions and adblockers? No.
Straight from the page itself:

Non-goals:

...

• Enforce or interfere with browser functionality, including plugins and extensions.

Edit: To elaborate on the consequences of the proposal...

Could it be used to prevent ad blocking? Yes. There are two hypothetical ways this could hurt adblock extensions:

1. As part of the browser "environment" data, the browser could opt to send details about whether built-in ad-block is enabled, any ad-block extensions are enabled, or even if there are any extensions installed at all.

Knowing this data and trusting it's not fake, a website could choose to refuse to serve contents to browsers that have extensions or ad blocking software.

2. This could lead to a walled-garden web. Browsers that don't support the standard, or minority usage browsers could be prevented from accessing content.

Websites could then require that users visit from a browser that doesn't support adblock extensions.

I'm not saying the proposal is harmless and should be implemented. It has consequences that will hurt both users and adblockers, but it shouldn't be sensationalized to "Google wants to add DRM to web pages".

Edit 2: Most of the recent feedback on the GitHub issues seems to be lacking in feedback on the proposal itself, but here's some good ones that bring up excellent concerns:

• Browsers developed and distributed by large tech firms have a conflict of interest with holding back or limiting attestation. Attestation enables the web to be restricted in a way that benefits tech firms. For example, Office 365 could require that it is used only on Windows and/or only through Edge.

• Similarly to what I brought up, having the ability for websites to trust a (browser, os) tuple could allow for certain browsers to be preferred, simply because they do not support extensions.

• How it will create hostile discrimination, and two-tiered services based on whether browsers are attested or not.

• The proposal does not do an adequate job explaining how a browser may be attested to.. Would this require something like Secure Boot in order for a browser to be attested to? That would discriminate against users with outdated hardware lacking support for boot integrity, or users who don't have it enabled for some reason or another.