Apple seems to have made an error when listing which iPhone and iPad models were capable of playing music featuring Spatial Audio through their built-in speakers, based on a recently updated support page. Taking away a feature is not a welcome direction, however, correcting a mistake that has been found is necessary and this is most likely what happened, rather than Apple purposely limiting older devices.
Apple's Spatial Audio works in conjunction with music, movies, and TV shows that are encoded with Dolby Atmos, providing a high-fidelity sound that has more dimension. Dolby Atmos compatible hardware can simulate directional audio that goes beyond the previous generation of surround sound technology to include height, meaning a scene that includes a helicopter flying in from the side and then zooming over the camera will be represented that way in the audio soundscape. Spatial Audio considers the listener's movement when used via AirPods Pro and AirPods Max, adding another level of immersion.
As noted by MacRumors, Apple's original list of devices that could play Apple Music's new Spatial Audio feature included the iPhone XR and iPad 6, both released in 2018, as well as the iPad mini 5, iPad 8, and third-generation iPad Air, which launched in 2019. Apple's updated support page shows a smaller list of compatible built-in speakers, still including a few devices from 2018. The new list of supported phones starts with the iPhone XS and XS Max, proceeding through the iPhone 11 and 11 Pro, to the current model, the iPhone 12, and 12 Pro.
iPad Speakers & Spatial Audio
Unfortunately, the list of supported iPad models has been reduced more than what befell the iPhone, with the fourth-generation iPad Air, and some iPad Pro models, being the only tablets whose built-in speakers have the ability to play Spatial Audio. Every iPad Pro 11-inch and the third generation or later iPad Pro 12.9-inch are supported. While this isn't good news for owners of the recently excluded devices, it should be noted that the best experience of Spatial Audio comes while wearing headphones or earbuds.
An easy solution for those that have older devices is to simply plug in a pair of wired earbuds and switch Dolby Atmos to 'Always On' in the Settings app under the Music tab, as noted on Apple's updated support page. Spatial Audio is a great way to experience audio with more dimension and several Apple devices are supported but, sadly, not as many as Apple originally listed.