What is the bug or the crash?

I've saved a few projects with current master (3.43) and these projects can not be loaded in previous versions.

Projects are loaded, but the project CRS is lost.

project = QgsProject.instance()
project.crs().authid()

Steps to reproduce the issue

Save project with current master

Read project with previous version 3.34 or 3.40

The project CRS is lost

Versions

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response