...

We discourage the use of localhost redirect URLs. Only in cases when you are developing, or when you use a desktop app, we support localhost redirect URLs. Since this traffic never leaves the host machine, the http:// scheme is allowed here.
You may use http://127.0.0.1 or the ipv6 equivalent, http://[::1]. Port numbers are allowed too. On production, you should only use this for native desktop apps.

Native apps

On mobile apps, we advise you to register application specific URLs for your redirect URL (in stead of using localhost). For Android, you can check https://developer.android.com/studio/write/app-link-indexing. For Apple, check their documentation on https://developer.apple.com/documentation/xcode/allowing_apps_and_websites_to_link_to_your_content

...