Garbage I reckon.
I can't find any legislation whatsoever about displaying an old or out of date tax disc being an offence. There is no way that the location which used to be used for a tax disc can now be considered an obstruction to forward vision. There are offences of failing to maintain glass or other transparent areas causing a detrimental effect on vision, but these are for situations like where there are no wipers fitted, or your washers are empty and you're driving around with a screen full of dirt.
Note .... My opinion only.
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