Permanent links to the CASA release notes:
https://casadocs.readthedocs.io/en/v6.5.3/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.5.0/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.4.4/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.4.3/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.4.0/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.3.0/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.2.1/notebooks/introduction.html
https://casadocs.readthedocs.io/en/v6.2.0/notebooks/introduction.html
https://casa.nrao.edu/casadocs/casa-6.1.0/introduction
https://casa.nrao.edu/casadocs/casa-5.6.0/introduction
https://casa.nrao.edu/casadocs/casa-5.4.1/introduction
https://casa.nrao.edu/casadocs/casa-5.4.0/introduction
https://casa.nrao.edu/casadocs/casa-5.3.0/introduction
https://casa.nrao.edu/casadocs/casa-5.1.2/introduction
https://casa.nrao.edu/casadocs/casa-5.1.1/introduction
https://casa.nrao.edu/casadocs/casa-5.1.0/introduction
https://casa.nrao.edu/casadocs/casa-5.0.0/introduction
Earlier versions can be found in
https://casa.nrao.edu/previous_casa.shtml