This is disappointing. I was using XSLT for transforming SVGs, having discovered it early last year via a chat. Even despite browsers only shipping with v1.0 it still allowed a quite compact way to manipulate them without adding some extra parser dependency.
I wonder what the next step of removing less-popular features will be. Probably the SMIL attributes in favor of CSS for SVG animations, they've been grumbling about those for a while. Or maybe they'll ultimately decide that they don't like native MathML support after all. Really, any functionality that doesn't fit in the mold of "a CSS attribute" or "a JS method" is at risk, including most things XML-related.
[0] https://news.ycombinator.com/item?id=43880391
Which is miles better than having to having to use calcs for CSS animation timing which requires a kludge of CSS variables/etc to keep track of when something begins/ends time-wise, if wanting to avoid requiring Javascript. And some years ago Firefox IIRC didn't even support time-based calcs.
When Chromium announced the intent to deprecate SMIL a decade back (before relenting) it was far too early to consider that given CSS at that time lacked much of what SMIL allowed for (including motion along a path and SVG attribute value animations, which saw CSS support later). It also set off a chain of articles and never-again updated notes warning about SMIL, which just added to confusion. I remember even an LLM mistakenly believing SMIL was still deprecated in Chromium.