Apple has an updated chart showing how the new $4.99/month plan differs from a regular Apple Music plan:
From reading the press release, it seems that the Music app will offer a stripped-down experience for these users:
Apple Music Voice Plan subscribers will get a customized in-app experience with suggestions based on the listener’s music preferences and a queue of recently played music through Siri. Within the app there will also be a dedicated section called “Just Ask Siri” where subscribers can learn tips to optimize Siri for Apple Music.
For access to Apple Music’s premium offerings, including Spatial Audio and Lossless Audio, Lyrics, Music Videos, and more, subscribers can easily switch to Apple Music’s individual plan for $9.99 per month or family plan with up to six accounts for $14.99 per month at any time.