O-34 - Neutron stars with gigayear lifetimes, fields of 10^8 G and up, and long-lived binary companions are observed
The neutron-star side of the empirical anchor: their extreme density (>
Link to original2 x 10^14 g/cm^3, d_0 <0.01 cm stopping scale) makes them stop even the black holes that white dwarfs cannot (D >= 8), and their known ages bound how long a captured stable black hole can have been accreting. The strong magnetic fields are a complication (synchrotron screening of charged cosmic rays), which is why the binary-companion beam-dump geometry and the cosmogenic-neutrino flux carry the production side of the neutron-star bound.
Why this is evidence
Neutron stars stop even the holes white dwarfs cannot (the higher-dimension cases), so their observed gigayear lifetimes — with binary-companion beam-dump geometry supplying the production side despite magnetic screening — close the branch of fast-accretion configurations the white-dwarf bound leaves open. H-22 predicts these old neutron stars exist; under H-8-style catastrophic accretion physics, a captured hole would consume or disrupt a neutron star quickly, making the observed population improbable. H-36 intermediate regimes on sub-gigayear timescales are likewise disfavored. Non-obvious because the discrimination runs through the capture-and-accrete chain rather than anything the observation states about trapped holes.