it is developed ‘long ago’ i’d think when openmoko is developed but for a while it has been ‘forgotten’ but that it is actually heavily used these days!
the reason is simply because dfu-util supports the stm32’s version of dfu (it can flash firmware onto stm32 mcu’s with the mcu’s internal bootloader) http://dfu-util.sourceforge.net/dfuse.html
in particular it is a tool of choice for arduino sketch / firmware install for stm32duino libmaple core
stm32duino is ‘going places’ https://trends.google.com/trends/explore?date=all&q=stm32duino
i’m not sure why google trends only show the statistics for usa and germany but the trend is pretty clear
it makes sense to support its tools in the standard distributions e.g. dfu-util which its use isn’t limited to stm32 alone
mcus like arduino, and stm32 is having a ‘revival’ as these boards affectionately called the ‘blue pill’ https://www.ebay.com/sch/i.html?_from=R40&_nkw=stm32f103c8t6&_sacat=0
are pretty common and low costs, in particular stm32f103c8 blue pill boards (and the former maple mini) are very popular
and pretty importantly stm32f103c8 is a generic usb device, you can re-program it into any usb device class (usb-serial, hid keyboard, mouse, audio, camera, oscilloscope, etc etc) if you are happy to write code