Legacy context usage prints warnings in React StrictMode

I noticed when using [rheostat](https://github.com/airbnb/rheostat) that this project is printing warnings in React StrictMode because it's using the legacy React Context instead of the one released in React 16.3. I understand this library would like to continue supporting React <16.3, but it also prevents using concurrent mode when that is eventually released into stable.

What is the long term strategy for releasing this library? Should there be a new major version/forked library which supports React 16.3+? Is it possible to feature-detect the new Context API and use that if it's available?

The issue names no files, tests, or entry points. First clarify the long-term compatibility strategy for React versions below and at 16.3, including whether a major version or feature detection is expected; done means an agreed direction and defined release scope.

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