[QGIS-Developer] QGIS 3.44 projects compatibility with previous versions

[Stefanos Natsis]

Hi list

During the last dev cycle, a change was introduced in the way that
properties are stored in the .qgs xml file
https://github.com/qgis/QGIS/pull/60855

While this solved a real issue (data-driven properties could result in an
invalid property name with a leading digit) in an elegant way and is
technically backwards compatible (projects generated with older versions
can be opened without issues), it has the side effect that projects stored
with 3.44 will not properly load in older QGIS versions (stored properties
are ignored and also project CRS is undefined). Plugins could also
malfunction as they often use project properties for storing their
project-related configurations.

One potential solution would be to backport this to LTR, however from my
experience a good percentage of the users use older QGIS versions and even
those on LTR do not always upgrade as soon as a new release is out, so
backporting could eventually make things worse!

With QGIS 4.0 around the corner, imho, it would be better to defer this
change for a couple more months and only include this kind-of-breaking
change in QGIS 4.0

I briefly chatted with Jurgen at the hackfest about this and while
acknowledging the issue, was not super happy to revert, so I'm interested
in gathering more opinions here.

Best
Stefanos
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