You should develop against the master branch. The 1.8 branch will (generally!) not have new features added, and will only include significant bugfixes. If you develop against the master, then once your changes get merged, they will be available for use in the 1.9 binding which can be run on 1.8.
I’ve not looked to see if there’s an Insteon binding for OH2 yet, but if not, and the OH1 binding is still used, then the changes will also be usable there.