You can absolutely publish apps that target versions of iOS prior to iOS 18; eg all my apps are iOS 15+, I published one as recently as last week.
You can build an app with the iOS 18 SDK that target previous versions just fine (as long as your calls to APIs posterior to the version you’re targeting, if any, are wrapped in the proper macros) and submit them to the App Store.
So yes, you have to build your apps with the latest SDK to publish them - but that doesn’t restrict the target OS version in any way (ie I can build apps that target iOS 9 with the iOS 18 SDK).
Where it starts to be a pain for new apps built in 2025 is if you want to target pre iOS 13, as you’ll have to use the older AppDelegate mechanism, but still totally doable.
(That’s getting deprecated with the next release, but deprecated APIs still build just fine).
You can absolutely publish apps that target versions of iOS prior to iOS 18; eg all my apps are iOS 15+, I published one as recently as last week.
You can build an app with the iOS 18 SDK that target previous versions just fine (as long as your calls to APIs posterior to the version you’re targeting, if any, are wrapped in the proper macros) and submit them to the App Store.
So yes, you have to build your apps with the latest SDK to publish them - but that doesn’t restrict the target OS version in any way (ie I can build apps that target iOS 9 with the iOS 18 SDK).
Where it starts to be a pain for new apps built in 2025 is if you want to target pre iOS 13, as you’ll have to use the older AppDelegate mechanism, but still totally doable. (That’s getting deprecated with the next release, but deprecated APIs still build just fine).