C4::Letters::GetPrintMessages is a very basic wrapper around a simple SQL lookup. We could easily remove it and use Koha::Notice::Messages->search directly instead resulting in Koha::Notice::Message objects in the return.
The reason to do this would be to use the methods introduced in bug 35285 so we're consistent with how we output print notices in the browser from the staff client and more directly as the output of gather_print_notices.