Good suggestion, I’ll have a look if I can add a option for that to the InnoSetup uninstaller (with a big warning), by default it should just leave the user alone.
Did a quick peek into the InnoSetup documentation, it isn’t supported out of the box, the uninstaller doesn’t support custom page like the installer does. The only option would be to ask the user via a popup to remove all files. I don’t want to go that way, I think it is too risky that people chose wrongly and loose their data.
For people who really want to remove everything we can add a complete uninstall section to the manual which describes where data is created by Traintastic.