How long has it been since the app stopped functioning properly? Does it fail in that it won't even load, will load but won't connect to the guitar, or that just some functions not longer work?
A detailed look through the tech specs of the guitar on those websites selling it and looking at the manuals available on the GTRS website, there is absolutely no mention of which versions of Android (or iOS) are required for app compatibility. Therefore IMO it is not unreasonable to assume that the device OS version doesn't matter for the app to work. Also given that most people's mobile devices are used for lots of far more security conscious apps which often rely on the OS being updated, it is (again IMO) unreasonable to expect people not to update the OS on their device, especially since for security reasons most have their set to install automatic updates.