  Ocean-vaporizing impacts of chemically reduced planetesimals onto the early
Earth have been suggested to catalyse atmospheric production of reduced
nitrogen compounds and trigger prebiotic synthesis despite an oxidized
lithosphere. While geochemical evidence supports a dry, highly reduced late
veneer on Earth, the composition of late-impacting debris around lower-mass
stars is subject to variable volatile loss as a result of their hosts' extended
pre-main sequence phase. We perform simulations of late-stage planet formation
across the M-dwarf mass spectrum to derive upper limits on reducing bombardment
epochs in Hadean analog environments. We contrast the Solar System scenario
with varying initial volatile distributions due to extended primordial runaway
greenhouse phases on protoplanets and desiccation of smaller planetesimals by
internal radiogenic heating. We find a decreasing rate of late-accreting
reducing impacts with decreasing stellar mass. Young planets around stars
$\leq$ 0.4 $M_\odot$ experience no impacts of sufficient mass to generate
prebiotically relevant concentrations of reduced atmospheric compounds once
their stars have reached the main sequence. For M-dwarf planets to not exceed
Earth-like concentrations of volatiles, both planetesimals and larger
protoplanets must undergo extensive devolatilization processes and can
typically emerge from long-lived magma ocean phases with sufficient atmophile
content to outgas secondary atmospheres. Our results suggest that transiently
reducing surface conditions on young rocky exoplanets are favoured around FGK-
stellar types relative to M-dwarfs.
