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.
Your old job's broken workflow is not a good reason for keeping a fundamentally broken protocol that relies on allowing Remote Code Execution as a privileged user around.
I used to work in a web dev job where when they brought in "time tracking" they wanted everyone to update a spreadsheet with what they were doing every half an hour. A spreadsheet, as literally a .xls, on a shared Windows drive. Everyone spent more time waiting for access to the spreadsheet than they did doing any work.
This situation persisted for about two weeks, and the manager that came up with the genius idea about two weeks longer than that, before we eventually told the other managers we were downing tools and leaving if he didn't either get "promoted to customer" or lay off the charlie during work hours.
Because it's was a shitty company and I only worked there for one month. I absolutely hate any type of time tracking or attempts to micro manage.