Thanks for the quick reply. I think I stopped the in-depth review after stumbling upon the first two possible GPL issues, especially in datamatrix.c and the pdf417.c files. It seems like compared to the copy I have reviewed (as part of zxing-cpp), the Unicode license for the encodings has been added in the meantime. Besides this, the following aspects might be worth to consider as well: raster.c has the method draw_mp_circle, which is attributed as GFDL-1.2. Having a documentation license used for...
Possible GPL-2.0-or-later code in backend code