Their point is that the maintainer did not sign a contract that requires them to perform maintainer duties. They can choose to stop doing it at any point. They can choose to axe a feature that you deem essential. They can choose to rewrite the project in COBOL for the fun of it.
You may not like it, but that is how it is.
The only legal document involved is the license and any open-source license I've seen so far, has stated that the program is provided as is.
This is the license under which rsync is provided: https://www.gnu.org/licenses/gpl-3.0.en.html
See sections 15 and 16.
The only way you get to have a say in the matter, is by forking and becoming a maintainer yourself.