You won't be able to use an iPad as a home hub to control HomeKit accessories in the newios 16. The change was spelled out in the text found in the second developer version of the operating system.
According to the text in the Home app, a home hub is needed to take advantage of features like receiving accessory notifications and allowing other people to control your home. Until shared homes are upgraded to the latest HomeKit, you won't be able to view them.
“Only Apple TV and HomePod are supported as home hubs.”
If the change goes into effect, you will need to use a HomePod, a mini, or an Apple TV as your home hub when you upgrade to the newest version of the operating system. A request for comment was not immediately responded to by Apple. According to MacRumors, Apple is planning to remove support for iPad in the upcoming version of its operating system.
There is some good news on the smart home side of things. A new categories section and a redesign of the home tab will be included in the update. Matter, the new smart home connection standard with broad industry support that is set to arrive sometime this fall, will be supported by a later update to the OS. The upcoming Matter support may provide a clue as to why Apple is dropping iPad home hub support.