This is the United States subcategory, and the place in question is in the U.S. There’s no global standard for a particular delimiter in place names – far from it.
In the case of Derry/Londonderry, mappers have had to add a semicolon-delimited alt_name just so Nominatim will find it by both names.
I disagree with the implication that replacing the semicolon with another delimiter or truncating the result at the semicolon is a blue-sky idea. To the contrary, styles based on the Mapbox Streets source replace the semicolon in this way with an em dash.
When GraphHopper tells you to turn onto this road, it replaces the semicolon with a comma:
The Mapbox Navigation SDK puts the names on different rows and says just one of the names aloud for brevity.
This is within the realm of possibility: since 2014, openstreetmap-carto has been replacing semicolons in ref with a newline. However, the maintainers declined to pretty-print semicolons in name based on the opinion that name should not have multiple values in the first place. We can see that this decision has backfired, encouraging mappers to add multiple values anyways but in unstructured form.
For sure, it would be a monumental task to persuade mapping communities in multilingual regions to change their agreed-upon name delimiters, but that’s off-topic here. I’m not aware of any entrenched standard in the U.S. This is an opportunity to establish the semicolon as the standard delimiter for the relatively few cases where name needs multiple values in this country.
The only reason why a slash has become so prevalent in U.S. place names over the past month or so is that one mapper unilaterally applied it across the country. They’ve also been chided by Canadian mappers for the same practice.