That's the definition of a major version change though...
A major version change may (or may not) have some breaking changes, but most good libraries have migration guides and documentation on how to upgrade. Presumably the parent comment was annoyed that the breaking changes also included no docs or guides.
Theoretically yes, but if you think of the metric of % of library-interacting lines of code an average library user needs to change to upgrade, React Router has been a huge outlier compared to other libraries in the React ecosystem. Your routing components would change completely. And the maintainers saw no problem with this.
To be fair I believe I once read they now don’t do that but it was truly scarring to naively update to the newest version thinking my app would need a few tweaks to get going again, when in fact big chunks needed rewriting, with zero help from the developers.