It's not something we can add to the base runtime unless we do the same for other runtimes, for the sake of consistency.
Note that skeleton.SetSlotsToSetupPose actually does 1. reset draw order 2. reset slot attachments 3. reset slot colors.
So it would take more than an optional bool to handle all cases and disambiguate.
But yeah, spine-unity has a bunch of extension methods that do extra stuff anyway.
For your case, the method would go:
public void SetSlotAttachmentsToSetupPose (Skeleton skeleton) {
foreach (var s in skeleton.Slots) {
var data = s.Data;
if (data.attachmentName == null)
s.Attachment = null;
else {
s.attachment = null;
s.Attachment = skeleton.GetAttachment(data.index, data.attachmentName);
}
}
}
or if you are using Unity.
foreach (var slot in skeleton.Slots) slot.SetAttachmentToSetupPose();