My guess is that a lot of the controversy is simply because this is one of the first times that a major web feature has been removed from the web standards. For the past 20+ years, people have grown to expect that any page they make will remain viewable indefinitely. It doesn't matter that most people don't like XSLT, or that barely any sites use it. Removing XSLT does break some websites and that violates their expectation, so they get mad at it reflexively.

As someone who's interested in sustainable open source development, I also find the circumstances around the deprecation to be interesting and worth talking about. The XSLT implementation used by all the browsers is a 25 year old C library whose maintainer recently resigned due to having to constantly deal with security bugs reported by large companies who don't provide any financial contribution or meaningful assistance to the project. It seems like the browser vendors were fine with the status quo of having XSLT support as long as they didn't have to contribute any resources to it. As soon as that free maintenance went away and they were faced with either paying someone to continue maintenance or writing a new XSLT library in a safer language, they weren't willing to pay the market value for what it would cost to do this and decided to drop the feature instead.