Firefox and Chromium had in-built FTP protocol handlers. I believe extensions could depend on this in such a way that the removal might break things for them, but general web content couldn’t—it was just an external link.

Safari and IE didn’t have integrated FTP: rather, they could hand FTP links over to the OS for something outside the browser to handle.

Now Firefox and Chromium can hand FTP over to the OS too.

XSLT, on the other hand, is integrated, a true part of the web platform. XSLTProcessor on any page, <?xml-stylesheet?> on XML files loaded directly… it works everywhere. The case is quite different from FTP.

If they remove XSLT, this will be the first time a major baseline-available feature has been removed. (One or two minor features have been removed: SharedArrayBuffer was temporarily dropped very shortly after landing across the board, for solid security reasons, taking up to 4 years to return with suitable limitations; and mutation events came in 2011 before quickly being replaced with mutation observers in 2012–2013, finally being removed in the last year or so; and the unload event which has been known to be unreliable and a bad idea for at least a decade is in the process of being removed. But all three of these are likely to be very minor in their effects, where XSLT removal just breaks your entire page.)