  Cosmic microwave background (CMB) experiments that constrain the
tensor-to-scalar ratio $r$ are now approaching the sensitivity at which
delensing---removing the $B$ modes induced by the gravitational lensing of
large-scale structure---is necessary. We consider the improvement in delensing
that maps of large-scale structure from tomographic line intensity mapping (IM)
experiments targeting $2 < z < 10$ could provide. Compared to a nominal
baseline of cosmic infrared background and internal delensing at CMB-S4
sensitivity, we find that the addition of high-redshift IM data could improve
delensing performance by ~11%. Achieving the requisite sensitivity in the IM
data is feasible with next-generation experiments that are now being planned.
However, these results are contingent on the ability to measure low-$k$ modes
along the line of sight. Without these modes, IM datasets are unable to to
correlate with the lensing kernel and do not aid in delensing.
