The Cygwin DLL is based on something called Newlib from BSD, but it contains considerable new code from the Cygwin project. As a whole, it is LGPL-ed, with a special exception that also allows static linking.
I missed that LPGL switch in 2016, but I obviously mixed that up somehow. We never could use any GPL sources in newlib, hence it was always far behind other libc's.
And the biggest problem gone seems to be the mandatory CLA.
But nowadays cygwin lost it's dominant role from a decade ago.
