Recently the UK supreme court ruled against IS bride Shamina Begum's attempt to return to the UK to fight her citizenship being revoked.
As I understand it, what happened here was that the UK wanted to disown Shamina Begum after she joined the Islamic State. The Universal Declaration of Human Rights held that she cannot made stateless, but Shamina Begum held dual nationality, making it possible for the UK to revoke her citizenship and make her another country's responsibility (in this case Bangladesh). This seems rather silly to me, because it makes it an arms race to see who can revoke her citizenship faster. On the other hand, why can't the UK just revoke Shamina Begum's citizenship even if she held no other citizenship, since she evidently identified as an Islamic State citizen? Could the UK have done so without violating the Universal Declaration of Human Rights?
Only thing I can see is that doing so would constitute de facto recognition of IS as a sovereign country, but even then I don't see why it would be a problem, given that there are other states the UK does not recognize that control territory, e.g. Taiwan.