The GPIO lines are used quite a bit for the Sonar board and in fact that takes up 14 of the GPIO lines. 4 of those lines I have discussed how to use them if sacrificing what they do for Magni is acceptable to your overall design. There is another board in development for what we call Magni Gold that uses 7 more of the GPIO lines.
So my un-offical to be confirmed answer is that only pin 27 and pin 28 which are not really official GPIO lines are unused. We don’t have a good way to explain yet how to use the 7 lines ‘reserved’ for the new board but that may be something we have to address.
It is a large documentain task but I am working on some way to present the GPIO line usage as this has come up before. If we just say where they are used it does not help those who want to use them. We need a decision on how to proceed and if we can offer those lines for general use or not Once it is presentable we will be posting the info on the Learn pages most likely on the page we have for discussing Raspberry Pi GPIO lines which is https://learn.ubiquityrobotics.com/doing_more/GPIO_lines.html