Currently the MusicBrainz website displays flag icons on certain pages. For releases which are tagged [Worldwide], it displays the flag of the United Nations (UN). However, the flag represents a specific organization which not every state in the world is a part of. The following non-UN states can be entered as the release county on MusicBrainz: Kosovo, Palestine, Taiwan, Vatican City, and Western Sahara.
Unfortunately, there is no single flag that represents the entire world and all of its states. One alternative is a generic globe symbol consisting of a circle with lines for the longitude and latitude: 🌐. Apple changed for icon for language and region settings from the UN flag to a blue flag with a globe symbol in 2017. This has the advantage of being a visually similar current icon. An alternative would be a blue globe on a white background, like the emoji.