Opting out – yes. However, whether a municipality could join Switzerland, Germany, or Austria (or XYZ) is not regulated. This would require negotiations. So far, no community has chosen to leave.
Discussion
I hope that one attempts it so we can see that negotiation process play out.
A thing I'm not sure about: If a community opts out but "nobody will take them", do they end up "stuck" in Liechtenstein, or do they get to become a sovereign of their own?