iPhone rotation lock missing from control center

iOS 18 had a redesign of Control Center, into 3 (or more) different ‘pages’ (“groups of controls”).

Verify that you look for the orientation lock symbol in the correct ‘page’. The navigation between these ‘pages’ is to the right side of the screen (♡ your most used controls, ⌂ optional Home controls, ♫ Media Playback, Connectivity, and optional other user defined categories).

Controls that you never use may be hidden through click-and-hold in an empty spot in Control Center, then use the (−) symbol on a control. They can be revealed again there as well [(+) Add Control]. Add Control allows for text search, e.g. “Orientation Lock” (or part thereof; “Ori” is enough on my phone to filter to the wanted one).

Use and customize Control Center on iPhone - Apple Support

Rotate your iPhone screen - Apple Support