The initialized field was added in:
commit 82f01ad786
Author: Alex Goins <agoins@nvidia.com>
Date: Wed Apr 10 13:48:02 2019 -0500
xsync: Add resource inside of SyncCreate, export SyncCreate
But it added this field not at the end of SyncObject. It may not have
been _usefully_ possible to create those from another extension prior to
that commit, but that's still an ABI-incompatible change.
(cherry picked from commit 194ba38728)