Begum’s lawyers, the judges said, also unsuccessfully argued that citizenship deprivation was “disproportionately applied to British Muslims of certain ethnic minorities” and “impacts detrimentally upon the relations between members of Muslim communities and others”.
Her lawyers are morally right though, and in fact that was what the Tories designed the Borders and Nationality Bill to do: being able to make into second class citizens the 6 million plus Brits of Colour (plus some Irish folks), who could be argued to have access to another citizenship. The point is that we are vulnerable to retaliation from the Home Office if we step out of line, in ways that other British people aren't. I, an immigrant, am only conditionally British in a way my partner isn't, despite the fact we both now hold British citizenship.
Begum was groomed and trafficked as a child (she was 15), and really should retain her citizenship and be allowed back. If she has to be held to account for anything (and again, remember she was groomed online at the age of 15), she should be held to account here, rather than the Home Office making her stateless.
Other countries have taken back - and then sometimes prosecuted - adults in similar situations, so it's not as if there isn't an existing template for what to do, even if the Home Secretary wanted to disregard the fact that she was a child at the time - and of course children of colour are more often inappropriately viewed as adults in ways their white peers aren't.
More context of how other countries have handled similar situations: