Why FreeCAD 16
OSE deliberately uses FreeCAD 16 legacy, which can be downloaded as a FreeCAD 16 Appimage on Linux.
FreeCAD 16 is not the latest version, but it has a simpler workflow with an uncluttered part tree. v16 supports the main OSE workflow pf scalable design - such as Merge Workflow for large scale collaboration.
There is also a practical philosophy to not using advanced features - as it creates more responsible design by not using automated constraints. See more at Merge_Workflow#Philosophical_Point.
The main point is: it is easier for novices to learn, and OSE's goal is to have anyone become functional in CAD design within one hour from start. FreeCAD 16 accomplishes this, any higher versions make it more complicated with the extra items that appear in the part tree. OSE works a lot with the part tree (such as for Automated BOM Generation) so having it uncluttered is important.
We are not against power users doing FreeCAD 19 or the latest version. However, there may be file compatibility issues with v16, so there is a drawback to the latest version if our goal is scalable, crowd design and Public Engineering.