According to Kuo (who has previously been a fairly reliable source) the next iPhone 15 Pro models will not have buttons that move but instead have two additional Taptic Engines that provide haptic feedback when the buttons are “pressed”. Think of the Home button introduced on the iPhone 7 but for the volume and power buttons.
Solid-state buttons have a number of benefits including better water resistance, less moving parts for longevity, and maybe software enhanced features for consumers.
The standard iPhone 15 and iPhone 15 Plus are expected to have mechanical buttons but if rumours are true expect this feature trickling down to all iPhones in the future.
Apple will announce these new line-up of iPhones in September in 8 months time.