Is it definitely threaded? Just asking because some frames have holes dotted around for Di2 wiring, although not sure why there would be one there.
I know that some people add their own bosses therefor a frame bag - is it a gravel/touring setup?
If you are not going to use it I would put a bolt in it to stop any water ingress.Yes it is by and large set up for touring, relaxed geometry, mudguard eyelets, pannier holes so I think like you it is take some sort of bag clamping boss.
Personally will never use it, just to my memory have never seen one before.
I have enough trouble hauling myself around without adding any extra weight 😊