Well, the thing is, I'm not an Englishman so have no idea what's the deal with ‘Derry’ is, but I know what a shibboleth is. It has become an English term—and btw not an idiom in English since it has no other meaning in that language.
I had to look it up too: https://en.wikipedia.org/wiki/Derry/Londonderry_name_dispute
I just meant that idioms tend to be richer when the source is material.
Point taken on finding an example that everyone understands.
Any political context creates shibboleths by the dozen. The UK is good for making ones that stick around. "Nationist" and "National" political movements, for example, are apposite (and opposing).
Where I'm from, some people, having nothing better to do, occasionally raise noise saying that ‘to each his own’ is offensive because it happened to be written by Nazis on a concentration camp. I don't think I need any more unnecessarily politically aroused stubborn people around than there already are.
It's really only an issue in written form.
In spoken form, the London part is silent so they're both just pronounced Derry..
Signed,
An Irish person.