Do you remember that chrome lost FTP support recently? The protocol was widely used and simple enough.
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Edit: then account for the fact that this rare breed of content uploader doesn't use an FTP client... there's absolutely no reason to have FTP client code in a browser. It's an attack surface that is utterly unnecessary.
Dropping XSLT is about something different. It's not bad an in an obvious way. It's things like code complexity vs applicability. It's definitely not as clear of an argument to me, and I haven't touched XSLT in the past 20 years of web development, so I am not sure about the trade-offs.