Previous code in iOS 16 Beta 2 suggested that Apple’s iPad would not work as a Home Hub after the update, but The Verge reports that an Apple spokesperson said today that the iPad does still work as a Home Hub, but with one caveat: iPad does not support the new Home architecture coming out later this year.

The spokesperson said Apple is planning to “introduce a new architecture,” which could refer to Matter, a new smart home standard that will enable better interoperability between smart home devices from different companies and manufacturers. Apple is expected to roll out Matter support later this year.
iOS 16 and iPadOS 16 will continue to support iPad as a Home Hub with no loss of functionality. Along with these releases, the Home app will introduce a new architecture for a more efficient and reliable experience. However, iPad will continue to maintain and use its existing functionality.
Users using iPad for smart home setup will need to avoid updating to the new architecture, which will be available as a setup upgrade for the Home app in the iOS 16 and iPadOS 16 updates. The new Smart Home architecture is expected to be available in iOS 16.1 or iOS 16.2 software releases.
Apple has made it clear that the iPad will not be compatible with Matter, and the iOS 16 preview page states that Matter accessories rely on Apple TV 4K or HomePod as a Home Hub, with no mention of the iPad. so, in summary, for now, the iPad can be used as a Home Hub, but later this year, with a future iOS 16 update later this year, the situation may change.
Apple TV 4K, Apple TV HD, HomePod and HomePod mini can all be used as Home Hubs, and Apple is working on a new architecture.