XSL-FO: is XSL-FO dead technology and only used by niche companies?

I guess the only reason for the less availability of XSL-FO processors is due to the fact that it's formatting options are pretty large and quite complex too..

Since the amount of complexity involved, no one wants to sweat so much and make it open sourced. I heard there were quite good number of cool commercial processors available for XSL-FO. Even Apache FOP is a nice one to generate pdfs out of it.

Though i haven't used LuaTex, i found XSL-FO is a fun to work. Also it uses just XML to generate pdfs. If you ask me i will tell, it's simple yet powerful. And also i don't want it to die either.. :)

There is currently no free XSL-FO formatter besides FOP and a few others, which are (IMHO) not really usable. But that does not mean, XSL-FO is dead or so, it is used heavily in technical documentation.

Creating a full featured, standard compliant XSL-FO formatter is pretty hard, and FOP is not there yet. I'd guess that they are bitten by the pure complexity of the standard.

My company is currently creating another XSL-FO formatter with focus on high typographical output based on LuaTeX, so I know I bit of this area. It is not decided yet whether this will be open sourced or not. (Sorry that I can't help you there).

So my answer is: go with Apache FOP. Even if the last change is some time ago, it is pretty usable at this point. And XSL-FO is far from being dead. If you can afford it, use a decent XML Editor (such as OxygenXML) for editing, it makes editing XSL stylesheets fun. (I don't get any money by mentioning this.)

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.