From Bash website you can download “Bourne again shell (BASH)”.
Inside the download you can find the release notes.
EDIT: Or browse the source online, see NEWS for the summaries, and CHANGES for details.
For patch releases, these could be useful:
- https://ftp.gnu.org/gnu/bash/bash-4.4-patches/
- https://ftp.gnu.org/gnu/bash/bash-5.0-patches/
- https://ftp.gnu.org/gnu/bash/bash-5.1-patches/
- https://ftp.gnu.org/gnu/bash/bash-5.2-patches/