In fact it looks like there are still a few that use the registerSetting
API:
https://github.com/search?p=1&q=joplin+registerSetting&type=Code
Fixing it should be easy, it just means converting this:
await joplin.settings.registerSetting('mySetting1', { ... });
await joplin.settings.registerSetting('mySetting2', { ... });
To this:
await joplin.settings.registerSettings({
'mySetting1': { ... },
'mySetting2': { ... },
});