  We study future lepton collider prospects for testing predictive models of
leptophilic dark matter candidates with a thermal origin. We calculate
experimental milestones for testing the parameter space compatible with
freeze-out and the associated collider signals at past, present, and future
facilities. This analysis places new limits on such models by leveraging the
utility of lepton colliders. At $e^+e^-$ machines, we make projections using
precision $Z$-pole observables from $e^+e^-\to l^+l^- + $ missing energy
signatures at LEP and future projections for FCC-ee in these channels.
Additionally, a muon collider could also probe new thermal relic parameter
space in this scenario via $\mu^+\mu^- \to X + $ missing energy where $X$ is
any easy identifiable SM object. Collectively, these processes can probe much
all of the parameter space for which DM direct annihilation to $l^+l^-$ yields
the observed relic density in Higgs-like models with mass-proportional
couplings to charged leptons.
