I don't disagree that Google is killing the open web. But XSLT is a pretty weak argument for showing that. It is an extremely complicated feature that is very seldom used. I am very doubtful dropping support is some evil political decision. It is much more likely they just don't want to sink resources into maintaining something that is almost never used.

For the specific use case of showing RSS and Atom feeds in the browser, it seems like a better solution would be to have built-in support in the browser, rather than relying on the use of XSLT.

The sites that will be broken are disproportionately important though. Congress.gov/govinfo.gov, weather.gov, europa.gov, plus dozens of sites for libraries, and universities.

Looking only at how many sites use a feature gives you an incomplete view. If a feature were only used by Wikipedia, it'd still be inappropriate to deprecate it with a breaking change and a short (1yr) migration window. You work with the important users to retire it and then start pulling the plug publicly to notify everyone you might have missed.

Of course built-in support for RSS would be better. But what are the chances of that happening?