No actual changes, sort of a bug fix - documented way of requiring the package in CJS env was to require('react-textarea-autosize').default, but accidentally it got broken way over half a year ago in 5.1.0 (28 July 2017). Since that version CJS envs could do require('react-textarea-autosize') (without additional .default).
Interop between those 2 module formats is tricky if we do not provide additional interop helpers (that were omitted in "broken" versions) when bundling. It caused some issues in mixed SSR / browser development, so it got fixed, but due to long-standing nature of this "bug" it got decided to release this as major version if by any chance somebody has relied on the broken behaviour.