The heat shield between exhaust and petrol tank has come loose and it has been suggested that it can be discarded because it is not necessary. This seems surprising and comments would be appreciated.

It may only have cost Ford pence to fit it - but they must have had a reason - I'd suggest it's to reduce the rate of fuel evaporation.

it has been suggested that it can be discarded because it is not necessary.

All OEs scrutinise every single part of the engineering of their cars. Every item fitted has to be fitted for a reason and the cost justified. In this case as it's a non visible part it will be fitted for good engineering reason. Engineers probably found that under some circumstances heat from the exhaust may have an effect on the tank and testing will have proved the effectiveness of the shield. Ford clearly believed it was necessary.

In all probability you could remove it and suffer no ill effects, but this can't be known for certain.

I would suggest you try to come up with a fix. If the fixings are corroded or missing try adhering it back into place with some of this:
www.toolstation.com/shop/p60981?searchstr=stix