macOS High Sierra: still usable as a development environment?
This is not a critical problem. You should be able to continue developing on your MacBook Pro for a year or two yet.

iOS Development

For iOS development, so long as Xcode supports your operating system, there is no problem. Do check the iOS SDKs supported by Xcode in case you need to work with specific features.

macOS Testing

You will be able to continue developing applications that target macOS 10.13 and below. Building for macOS 10.14+ is possible but testing will be a problem.

Some features, such as Dark Mode/Appearance support, will be difficult to implement correctly. Implementing support does not require the developer run macOS 10.14, but they should be able to borrow or test on a Mac running macOS 10.14.

App Store Submissions

You will continue to be able to submit applications to Apple's App Stores. Historically, Apple have not restricted what tools or versions are used to build applications they accept.

Future

Be aware that Apple previewed an iOS on macOS framework at WWDC 2018. This suggests significant changes are coming to at least the Mac platform. You may want to wait until more is known, before investing too much in a new Mac.