React-Calendar 3.0 will not include TypeScript typings by default. This is because TypeScript popularity is still relatively low comparing to JavaScript community, and it became a major slowdown for React-Calendar's development. I'll be happy to see typings created in DefinitelyTyped.
Last tree with typings for reference: https://github.com/wojtekmaj/react-calendar/tree/6905e6998f66b1d976eec99d5369374390106208
PR created on DefinitelyTyped: #43540
Wooohooo! You're a hero!
PR got merged - thank you once again @Guymestef!
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PR created on DefinitelyTyped: #43540