I'm cleaning off my desk and just found an old dog-eared copy of O'Reilly's XSLT pocket reference. I referenced in frequently ~2006-10 but haven't touched in awhile. It's got me wondering... when did you (the HN commenter) last use XSLT?
Also, are there any actively used APIs (obviously SOAP, but also REST) that only return XML?
To answer my own question: My last big XSLT project was for a video curation app startup back in 2010-ish. When the iPad came out and every company needed an app, you could proxy app calls via Charles to access unprotected (and unstandardized) XML feeds of video content. For the better part of a month I downloaded every app with video, looked for accessible API feeds and wrote XSLT transformations and XPath to standardize for consumption by our app. At times it was rote, but I learned you can gleam a lot about a company when you inspect their internal APIs.
I use it sporadically, a few times a year when I have some XML to munge and I do it with XSLT + xsltproc. For a certain class of simple transformation, it beats writing something in python / java / whatever.
At work in 2011 but only because a colleague was really into it and could smash out a good XSLT in an hour, so I ended up using that and making modification (maintaining it). The XSLT was for taking really awful responses from an API and making them more palatable.