Actually, no, this information is not saved anywhere. The date that the post was last updated in is the post database, but I don’t know this is updated when custom fields are updated https://codex.wordpress.org/Function_Reference/get_the_modified_date. First you need to do some testing to see if it’s updated and then you need to figure out a way to get the value outside of the loop because all of the function that exist for getting this value can only be used inside the loop.
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