Reminder: Alias locales are defined from Unicode Common Locale Data Reposotiory (CLDR) which is loaded by MusicBrainz Server through the package DateTime::Locale.

The latest version 1.35 of this package is built from CLDR 40, whereas the currently used version 1.25 of this package is built from CLDR 36, according to the changelog.

Basically, the list of locales is updated with some locales being renamed, many being added (Belarusian, Dogri, English Maldives, Fulah Adlam, Hindi Latin, Kaingang, Maithili, Manipuri, Malay Indonesia, Norwegian, Nigerian Pidgin, Sanskrit, Santali, Sardinian, Sundanese, Unknown language, Nheengatu), some others being removed (Church Slavic, English United States Computer, Prussian, Root, Volapük). See release notes of CLDR 37, 38, 39, and 40 for details.

In the database, there are currently 13 aliases only that must have their locale updated to work with CLDR 40, so it isn't too bad. The exact list will be added in comment.