Restrictions were not introduced to "save" people, in such a way that you could restrict the lockdowns to the more vulnerable elderly population. Such a lockdown would have to be draconian (China-style).
Rather, restrictions were introduced (except for Australia and various countries in Asia which had zero case policy) to curb the growth in the occupation of hospital beds before the hospitals were full. Up until the introduction of vaccines and more contagious variants (Delta/Omicron) there were clear effects on Rt from the introduction and lifting of restrictions. Reduction of the reproduction rate cannot be achieved with a lockdown only of >60 year old people, since (like vaccination) the effect is inversely proportional to the amount of people not being locked down.