If you're targeting PowerPC, and using Symantec, you must use version 8 or higher. If you're ok running M68k code in emulation, anything goes.
The problem was Symantec was late for the boat to PowerPC. When 8 finally did come out, it was buggy, had far larger requirements, and was a shocking contrast to the slim, fast, tiny compilers they'd been known for.
Review in MacWorld:
https://archive.org/details/eu_Macworld-1995-07-INT_OCR/page/n65/mode/1upAfter that they spent a lot of time catching up, but never hit the peaks of greatness they had with their prior products. They were much more focused on Java and Windows, maybe. They never bothered to support Carbon, from what I can tell,.
I wouldn't, personally, recommend them for PowerPC, doubly so for MacOS 9, for some issues people have had, and support for OS and C++ language features lagged CodeWarrior, and even MPW, quite a bit. Not that it's bad at all, but you'll probably have a much better time without fighting the compiler itself. For MacOS 9, CodeWarrior is hard to beat. MPW is extremely powerful, but not user friendly or a traditional IDE, in any sense.
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With that said, completely different story for M68k development. Symantec C++ 7 and earlier Think C's are amazingly fast and tiny, and ideal for System 7.5. If you're doing System 6 development, and don't need C++, Think C 5 will run on a Plus. If you compare them with Symantec C++ 8.x, it's like night and day. But that's not what we were talking about...
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The original question was about moving older projects to newer systems. As far as the MacOS API goes, it's a thin line between the "core" ToolBox and Extensions that got added on, and became standard parts of the OS.
You can add things to 7.0/7.1 that were standard in 7.5/7.6. Same with 7.5/7.6 to "catch up" to 8.0/8.1. The leap from 8.0/8.1 to PPC only 8.5/8.6/9.x was larger, with many new ToolBox UI calls. There wasn't as big a jump between 8.5/8.6 and 9.x, especially from a programming standpoint.
Programming for them is similar, in leaps and what is included. If you require Appearance, for example, you don't necessarily require 8 or higher, as your users can add the Extension to earlier systems. Same with OpenTransport, and many more things. You can even use early versions of Carbon on 8.1.
The core API didn't change radically over the years, and so System 7 knowledge still works well on MacOS 9, to a point. Going from M68k to PowerPC will require some dealing with the Mixed Mode Manager, and Universal Proc Pointers for callbacks. So that is probably the biggest leap for very old code.