is it true that xcode 15 apps won't compile in xcode 16?

NO.

I have several apps that were developed in various versions of Xcode.

And compiled on Xcode 15, they compile without issue on Xcode 16.

What could happen is that you get some warnings that some API will be obsoleted. But that's something very common that any serious developer knows how to solve easily.

So your developer may be trying to fool you, just to ask for some extra cost or it did something really weird in its code.

My advice: Ask him/her to send you a screenshot with the error and the involved code when trying to compile on Xcode 16. And post it on the forum.

There's some issues with Xcode 16 for me where it will %100 time crash attempting to build a large complex project, and that is preventing it being possible to build. However hopefully that will be fixed by the time Xcode moves from beta to release.

"I find it difficult to understand why apple would want to make your app obsolete and force you to re-create it because of an update" They would not.

But what does your developer mean by "need to rebuild the entire app" and "re-create it".

Rebuild it / re-create it how, by doing what exactly? What are they saying is needed to be done to it?

just an extra information since I can't edit the post.

image below is the errors the developer sent.

Also, what he said: "the project is using so many dead libraries and Apple won't let us upload using Xcode 15 next year"

I understand now what your developer is saying. iOS apps are developed using Apple's programming language Swift. There's a new version of Swift available in Xcode 16, Verizon 6. Your developer is saying you need to migrate your app's code to Swift 6 and/or saying that the app cannot continue to use some libraries unless they too are updated by their developers to support Swift 6 as they won't build (or if they are dead and no longer being maintained then to find alternatives).

Apple should not be forcing people to migrate to swift 6, and Xcode should continue to build with apps built using an older version of Swift and therefore the fact that does not occur in your case is perhaps a bug in Xcode and might get rectified in the next release. Something else to try as a temporary workaround is to make sure that the Xcode setting Treat Warnings as Errors is set to NO to see if that makes the errors go away.

What they are saying about not being able to upload with Xcode 15 next year is true.

I don't think your developer is trying to cheat you but Xcode 16 is still in beta status, a big decision on if things need re-writing/replacing should not be being made yet in my opinion until a release version of it has been made.

I found this thread about swift 5 and swift 6 warnings situation, the poster asks a valid question that nobody actually answers https://forums.swift.org/t/please-help-me-understand-the-decision-to-add-swift-6-warnings-to-swift-5/73417