0.48.17 is not enough to build geary. Alternatively we could revert those bits in geary but since 0.48.18 is in Debian unstable already so a backporting looks like the saner thing to do (or even making sure it's allowed into bullseye).
What would be the preferred way to get this done?
Asking for a backport of the current version in Debian testing (while writing this is 0.48.18-1) into bullseye backports or building up a own backport for byzantium?
Getting a newer version via bullseye backport would minimize any maintenance effort within the PureOS system.
I can handle this low priority for the moment, i found ways to build geary for the moment but i expect this issue to become more urgent during the byznatium cycle again (if not for geary then for other packages using vala). Issue here is that this will likely involve a glib upgrade as well.
I agree that doing this via bullseye-backports would be the right way to do this.