O-2 - Neutron stars with ages of 100 Myr to Gyr are observed intact
Secondary anchor bodies (density ~10^14 g/cm^3 stops any TeV black hole); the NS bound needs extra assumptions about cosmic rays penetrating NS magnetospheres, so G-M treat white dwarfs as the more robust case.
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Why this is evidence
Under H-7, cosmic-ray-produced long-lived black holes are stopped by NS density (~10^14 g/cm^3) and would accrete the star; 10^8-10^9 yr survival is then unexpected. Under H-4 survival is certain. Weaker than the WD bound (magnetosphere assumptions).