To release an app to the App Store or build one you need a Mac. So my sister offered me her 13" 2013 MacBook Air (128GB) for €200 (~$230). It's still running pretty well, so price-wise it's worth it (I guess).
But I heard that to use Xcode, you need the latest version, which is usually bound to the latest OS version.
Is this true? Do I need the latest OS version (High Sierra as of now) to develop and publish Mac/iOS apps?
Best Answer
The short answer is no, not necessarily.
Versions of Xcode, like any software, will have a minimum version of the operating system it requires to run. However, that isn't necessarily the latest version of the OS.
Some examples:
This highlights the crux of the assumption in your question - that you need the latest version of Xcode to develop and publish apps. However, this assumption is false. Developers do not have to use the latest version of Xcode to develop and publish apps to the Mac or iOS App Stores. In fact, you don't necessarily have to use Xcode at all.
Having said all that, the latest release version of Xcode (9.4.1) requires macOS High Sierra 10.13.2 to run, so in that sense it does require the latest major version of macOS (although not the latest minor version). But, as I said, you don't need to use the latest version of Xcode to develop/publish an app.