every article has two input fields in the publishing tab

Guess what both have the same field name="xreference"
The first is saved in the json field 'metadata'
The second is saved in its own dedicated field

I can't see anywhere that they are used in com_contact
the code for this was added in 1.6 by @infograf768

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contacts does not have an input field but there is one in the database.
This time there is some code that references it in the store function but I don't see how you could ever store anything if there is no input. This code was added with #29088 by @mbabker

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newsfeeds does have an input field and a database field

but i cant find any reference at all in the code to see if it is used

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There are various other places that get the field in a query for example but then dont do anything with it. To me this is all some legacy code that has lost the connection to whatever it used to do and should be removed. No point keeping and updating code that doesnt do anything