This has nothing to do with the “open web”. I don’t know if the people saying this just don’t have a meaningful definition of what open means or what. “Open” doesn’t mean “supports everything anyone has ever shipped in a browser”. (Chrome should support Gopher, really? Gopher was literally never part of the World Wide Web.)
What’s happening is that Google (along with Mozilla and Safari) are changing the html spec to drop support for xslt. If you want to argue that this is bad because it “breaks the web”, that’s fine, but it has nothing at all to do with whether the web is “open”. The open web means anyone can run a web server. Anyone can write a web site. Anyone can build their own compatible browser (hypothetically; this has become prohibitively expensive). It means anyone can use the tech, not that the tech includes everything possible.
If you want to complain about Google harming the open web, there are some real examples out there. Google Reader deprecation probably hurt RSS more than anything else. AMP was/is an attempt to give Google tighter control over more web traffic. Chrome extension changes were pushed through seemingly to give Google tighter control over ad blockers. Gemini in the search results is an attempt to keep Google users from ever actually clicking through to web sites for information.
XSLT in the browser has been dead for years. The reality is that no browser developer has cared about xslt since 1.0. Don’t blame Google for the death of xslt when xslt 2.0 was standardized before Chrome was even released and no one else cared enough to implement it. The removal of xslt doesn’t change the openness of the web and the reality is that it breaks very little while eliminating a source of real security errors.
> Google Reader deprecation probably hurt RSS more than anything else
And, indeed, if the protocol was one killer app deprecation and removal away from being obsolete, the problem was the use case, not the protocol.
(Personally, I don't think RSS is dead; it's very much alive in podcasting. What's dead is people consuming content from specific sites as a subscription model instead of getting most of their input slop-melanged in through their social media feeds; they don't care about the source of the info, they just want the info. I don't think that's something we fix with improved RSS support; it's a behavior issue looking for a better experience than Facebook, not for everyone to wake up one day and decide to install their own feed reader and stop browsing Facebook or Twitter or even Mastodon for links all day).