usually, if removal of a chimney breast results in any unsupported masonry above, then you will need to provide some kind of support, for said masonry. be it gallows brackets or wall-wall steel beam.
if you're lucky, and the whole chimney within the loft and above the roof line has been removed, then there will be no need for support.
the degree of strength within the supports will be determined by the severity of the load above. a shared gable chimney for example, which is a chimney built between you and the neighbour, and whereby removal of one of those chimneys creates an imbalance, then serious thought is needed when removing only one side of this type of chimney breast.
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