Would integrated hashtag make sense for EXIF Photo data?

I wonder if people have been placing hashtag somewhere in EXIF.

You could chuck it into description, but that may mean more processing time to extract hashtags.

Does EXIF have any 'hashtag' like capability of flexible human text tagging categories?

What would also be helpful is if we have a predefined taxonomy, which would be helpful for an AI to tag whats present in the scene as well. I'm pretty sure smartphones have enough power to at least do some cursory analysis if there is a dog in the scene or not (and the taxonomy will allow for searching for 'animals' under dogs).