Will the premier+ platform be able to get the firmware update that supports Apple play? And if yes, what is the target release date?
You probably mean AirPlay2/HomeKit.
Regardless, Roku hasnt announced exactly which models will get the support - just "select 4K Roku devices":
https://blog.roku.com/roku-os-94
Apple AirPlay 2 and HomeKit – Later this year, we’re expecting to deliver Apple AirPlay 2 and HomeKit capabilities on select 4K Roku devices. With AirPlay 2, you can stream, control, and share your favorite content directly from your iPhone, iPad, or Mac to your Roku device and bring the experience to the big screen. HomeKit allows you to easily and securely control your Roku device using the Home app and Siri on iPhone, iPad, Mac, Apple Watch, or HomePod.