As of BizTalk 2020 and Visual Studio 2019, when you click on the map grid, you get the map properties window, on which you can select various “XSLT Transformation Engines”.
BizTalk 2020 supports XSLT 3.0 via the Saxon transformation tool, which is created by Saxonica. Microsoft apparently chose the open source “Home Edition” (HE), and the current version is version 9.
Below are the editions of Saxon 9 provided by Saxonica, which I believe is Michael Kay’s company. He’s one of the founding fathers of XSLT, has pushed through new standards, and has written several book on XSLT and XPATH.
You can also write a .NET program, following an interface, and plug-in other XSLT transformation engines. If you do so, you have to create a small XML file to load other options in the above window.
When you create a brand new map (.btm file), the default option is “undefined”, which means that BizTalk will continue to run the original XSLT 1.0 transformation.
I’m still not sure what happens if you select “.Net Framework”.