The iPhone's volume buttons will no longer work with Spotify Connect
Briefly

Spotify announced that iPhone users will no longer control volume of connected devices with physical buttons. This change is a result of Apple discontinuing necessary technology.
When iPhone users press the physical volume rocker, Spotify informs them to use an in-app slider instead. This change is due to Apple's refusal to share technology.
Spotify states that this change is intended to provide 'persistent, high-quality' volume control, citing issues with volume spikes and instability when using physical buttons.
Spotify requested a solution from Apple to enable better control of third-party devices, similar to their offerings for HomePod and Apple TV, but was denied access.
Read at The Verge
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