Apple has confirmed that the forthcoming iOS 17 will require an iPhone XS or newer, although it has not said whether all features will be available on all models.
Apple's iOS 17 will run on the iPhone XS and later
The iPhone XS was launched in 2018, so in theory this means any iPhone released in the last five years will work with the new iOS. In practice, it's likely that support will vary, with some features requiring more recent models.
Previously, Apple support has said only that "Your features and apps may vary depending on your iPhone model, region, language, and carrier." Users have been directed to an Apple website listing feature compatibilities, but it tends to only list country- and region-specific differences. At time of writing, it has not been updated to give any details of iOS 17.
AppleInsider will be testing the betas, and further feature compatibility will be made clear over the course of the week.