This is the collision I noted earlier between the scheme for dialects and the scheme for geopolitical naming disputes. Since the usage is so far from universal among speakers of American English in the United States, an *_name:en-US of some sort (alternative? official? national?) would be more accurate than a plain name:en-US. The authorities haven’t managed to turn this into a Northern Irish situation yet. alt_name itself is basically nothing more than a list of search keywords, but it doesn’t hurt to further qualify it by language and country.
In case you’re wondering, the feature’s other alt_name comes from Vietnamese: older speakers in the U.S. have a starkly different name for “Mexico” than younger speakers in Vietnam. Vietnam’s education and foreign affairs ministries have still two more official spellings for the country’s name, but so far we haven’t bothered to tag them on this feature.
The Vietnamese Wikipedia recently voted to stop acknowledging most Vietnamese exonyms in favor of more “modern” English or the local language, leaving Wiktionary and OSM responsible for informing people about the exonyms that people actually use in real life.
Ironically, Spanish-language media keeps having to explain why Gulf of America is problematic in English. In Spanish, América refers to the Americas as a whole, not the U.S. specifically. If one didn’t know any better, one would think it’s a gift to Cuba! Of course, this doesn’t justify the attempt to rename the gulf, since the intention is abundantly clear, but it does leave the name vulnerable to others reclaiming it for themselves. A more erudite politician might’ve chosen Gulf of Florida, which at least has a historical basis, dating to around the same time as Gulf of Mexico. But here we are.