How should you properly maintain backward compatibility for a genuinely widely used published package while still continuing to actively evolve its own overall API design and add genuinely new features over time?

Maintaining backward compatibility for a genuinely widely used package typically means preferring to properly add genuinely new optional parameters or entirely new additional methods rather than modifying an already existing method's own established signature, properly using Dart's own @Deprecated annotation to clearly warn consumers about a specific method that will genuinely be removed in some future particular release well ahead of actually removing it, and generally reserving any genuinely necessary breaking changes specifically for a properly planned major version release, clearly and thoroughly documented within your own package's changelog, giving consumers genuinely sufficient advance time and clear specific guidance to properly update their own existing code.

@Deprecated('Use formatDateTime instead. Will be removed in version 3.0.0')
String formatDate(DateTime date) { ... }