I understand what stands behind the request, however there are all kinds of levels of "cross platform". The b4xlibs implemented in B4X are 100%.
However there are other important libraries that are also somewhat cross platform. For example iArchiver does share some of the API with Archiver. Same is true for iRandomAccessFile and ByteConverter.
I think that it is a mistake to limit yourself to B4X libraries.
You can check B4X Pleroma which is a complex cross platform app. It uses many B4X libraries as well as platform specific libraries. Where it made sense, I added classes that act as an abstract layer and hides the differences.
Eventually we are talking about different platforms and with different behavior and different user expectations.