We present a generic wave equation for surface plasmon polaritons on any macroscopically curved metal-dielectric interface, with isotropic and anisotropic geometric potentials linear in curvature. Remarkably, the anisotropic birefringence vanishes if the metal-to-dielectric permittivity ratio equals the golden ratio squared, mimicking isotropy at linear order on an actually anisotropic surface, as confirmed by full-wave simulations. Applying our equation to quantum emitters on metallic spheres, we demonstrate a curvature-controlled reshaping of collective decay rates and frequency shifts.