Relic shield offers relatively little for the tanky support aside from the healing proc and the base health. If you believe that the sustain is the problem, nerf the healing, but don't nerf the actual health value on it. Compared to spell thief and ancient coin (which has been eyed as getting a buff), 75 health > 50 health is a pretty substantial nerf to a support class that needs to be relatively tanky when compared to the other support archetypes. Relic shield has already been nerfed to make it less appealing to the ADC, please don't make it even less appealing for the support.
Sincerely, a predominantly tank support main.