No, neither dead nor niche.
The question in 2010 pointed to the previous FOP release being in 2008. As of 2018, FOP has made a release every year between 2015 and 2018 (see https://xmlgraphics.apache.org/fop/news.html). If you were to also look at commercial formatters, Antenna House makes a maintenance release roughly every 45 days (see https://www.antennahouse.com/antenna1/news-events/).
https://www.antennahouse.com/antenna1/formatter/ currently has links to samples of a car manual, an annual report, and a tax form. Train tickets in Germany are generated using XSL-FO. I'm sometimes surprised how widely it is used.
