As soon as a Python release enters beta phase, all dependencies are locked. Since Python 3.9.0-b1 supported --with-system-libmpdec with libmpdec-2.4.2, all future releases of 3.9.0 until EOL of 3.9 branch have to support 2.4.2. Since 3.9.0-b3 does not work with libmpdec-2.4.2 you have introduced a regression.

It's always possible to drop support for older versions of a dependency. A developer has to make a formal case for deprecation, provide technical arguments why an old version of a dependency cannot be support without great effort, and get written permission from the release manager. It's the release manager's discretion what they consider a regression. It's well in their power and authority to refuse and even revert a change if a release manager considers a change a regression. A developer can call upon the steering council to mediate.

Any kind of personal attacks on community members, core developers, or release manage is not acceptable way to resolve a dispute.