Landau damping has previously been shown to be the dominant nonlocal effect in sub-10nm plasmonic nanostructures, although its effects on the performance of plasmonic nanocavities are still poorly understood. In this work, the effects of Landau damping in sub-10-nm planar plasmonic nanocavities are analyzed theoretically, and it is shown that while Landau damping does not affect the confinement of the cavity modes, it decreases the quality factor 10-fold due to the introduction of extra loss for sub-10nm gap sizes. As compared to purely classical models, this results in a suppression in the Purcell factor by 10 fold, the spontaneous emission rate by almost two orders of magnitude, and the required oscillator strength to achieve strong light-matter coupling by two orders of magnitude as the gap is reduced to ∼0.5nm. Therefore, it is crucial to consider Landau damping in plasmonic-nanocavity design because it breaks the classical norm of achieving higher light-matter interaction strength in sub-10-nm gap-plasmon nanocavities.