Hi ohnonot,
After some research, you seems to be right. Here is what I found 
"The specification is downloadable from the Microsoft website, and the format can be implemented under a license from Microsoft. However, this license does not allow the distribution of source code and is not compatible with open-source licenses.
ASF was patented in the United States ([United States Patent 6,041,345] by Microsoft until August 10, 2019."
Source : Wikipedia
ASF is used in WMA/WMV files. It seems that these formats can’t be used freely in commercial softwares.