I don't necessarily disagree with you on principle, but in this specific case it's important to note that the security benefits of blocking extension modification via a dedicated API are very minor -- especially since extensions will still be able to inject JS into the page and read client information. Is it really important for me as an attacker to be able to modify your request when I can just edit the page directly, insert custom data, or steal your login credentials instead?

With respect, I would suggest that the cost-benefit analysis here is more one-sided than you think it is -- one-sided enough that an exploration of the security benefits would need to come with a lot of caveats, and would be a net decrease in the readability of the article.

It's not clear to me that deprecating a dedicated request modification API improves security in any meaningful, practical way for most users, and I think there's some benefit to occasionally skipping details that are irrelevant to the main story, so that ordinary people will find it easier to wade through.

I would compare this to the way that Google bundles wifi access and location access on Android. People complain about it, but it's a sensible decision -- there's no point in restricting just one of them. Similarly, I don't see any point to restricting request modification without also restricting access to request content/headers/cookies and removing the ability to inject arbitrary JS into the page. I think Google is engaging in security theater here.