During the transition from Tough Rubber and Vulcanised Rubber Cabling to the more modern PVC / PVC Flat Twin & Earth Cabling used nowadays, lighting cabling (1.5mm cross sectional area or less) was Twin and Earth as it was fitted with a CPC (Earth).
During the 1970s the vast majority of light fittings in houses consisted of either Bakelite, wooden or plastic ceiling pendants which did not require an earth and the regulations did not stipulate that lighting circuits had to be wired with one. As a result, the vast majority of early 70s house (and before) do not have a lighting CPC and this can be dangerous.
Even in an installation fitted with Class 2 fittings throughout, the chances are that there will be metal back boxes behind the light switches and these back boxes do need to be earthed (because they are in contact with the screw heads holding the switch on which are obviously accessible and could very easily become live). When twin cable was used the back boxes for lighting circuits had plastic lugs for the screws to attach into so the screw heads were insulated from the back box but these aren't made anymore.
Where this cabling is found the best solution is to re-wire the affected parts, particularly the switch drops. Where this is not practical, Class 1fittings (ie those requiring an earth) should never be fitted and if there are already Class 1 fittings installed these should be removed immediately. Only fit standard pendants or class 2 fittings (should be marked with an outline of a black square inside a larger one) and consult with an electrician immediately to discuss what can be replaced.