Sacred geometry is the tradition that holds geometric form and proportion to encode a divine order. Some of what it points at is straightforwardly true mathematics; some is true mathematics with an interpretation attached; and some is not true. Separating the three is the most useful service an account of it can perform.
True, and genuinely remarkable. There are exactly five regular convex polyhedra in three dimensions, tetrahedron, cube, octahedron, dodecahedron, icosahedron, and Euclid proves it at the end of the Elements. Not four, not six, and the proof is short. The golden ratio really does appear unavoidably in the pentagon, whose diagonal is exactly phi times its side. The hexagon really is the most efficient way to tile a plane with equal-area cells, a conjecture from antiquity that was finally proved in 2001. Phyllotaxis really does produce Fibonacci numbers in sunflower heads and pine cones, and the reason is a well-understood growth process rather than a mystery.
True, with an interpretation attached. Plato assigns the five solids to earth, air, fire, water and the cosmos in the Timaeus, and Kepler built a model of the solar system from them nested inside one another. Both are real historical positions held by serious people; neither is physics.
Not true, or not established. The golden ratio is routinely asserted to govern the Parthenon, the Great Pyramid, the human body and classical painting. The measurements do not support it: the claims generally depend on choosing which points to measure between, and the literature debunking them is substantial and specific. The golden ratio is mathematically rich without any of that, which is the frustrating part.
The tradition's real and demonstrable presence is in architecture and ornament, Gothic masonry, Islamic tiling, mandalas and yantras, where proportion was deliberately used to attune a space to a cosmic pattern.