While some mods claim to offer this, true offline downloads are a server-side function and are extremely difficult, if not impossible, to unlock with a client-side mod. Most successful patches cannot enable this feature.
For Android users who absolutely insist on a mod, the current "gold standard" is , not Revdl. xManager is an open-source manager that patches the official Spotify APK on your phone. It is safer because: spotify mod revdl