https://mail.kde.org/pipermail/umbrello-devel/2020-October/022723.html

> I propose extending the <XMI.exporter> tag to contain the
> UMBRELLO_VERSION_STRING, for example:
>    <XMI.exporter>umbrello uml modeller 2.31.70-a9d91d68
>                                 http://umbrello.kde.org</XMI.exporter>

umbrello/version.h
- In function umbrelloVersion() add " (experimental)" to the version
  string if the preprocesor symbols ENABLE_WIDGET_SHOW_DOC or
  ENABLE_XMIRESOLUTION are defined.
- On defining XMI_FILE_VERSION remove the #ifdef tests.

umbrello/umldoc.cpp
- In function saveToXMI1(QIODevice&) on creating the <XMI.exporter> tag
  append umbrelloVersion() onto the content.