/usr/share/glib-2.0/schemas/10_evolution-common.gschema.override is in evolution-common 3.28.1-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [org.gnome.evolution.mail]
prompt-check-if-default-mailer=false
|