They don't "allow" it, but most apps that need background execution just ask permission for geolocation tracking and pretend to use it, for example iSH[1]. There are a few activities that the app can do to prevent itself from being suspended when it goes out of focus, like playing sound, geolocation etc.
[1] https://github.com/ish-app/ish/issues/249#issuecomment-54433...