  The massive end of the gas-phase mass--metallicity relation (MZR) is a
sensitive probe of active galactic nuclei (AGN) feedback that is a crucial but
highly uncertain component of galaxy evolution models. In this paper, we extend
the $z\sim0.7$ MZR by $\sim$0.5 dex up to
log$(M_\star/\textrm{M}_\odot)\sim11.1$. We use extremely deep VLT VIMOS
spectra from the Large Early Galaxy Astrophysics Census (LEGA-C) survey to
measure metallicities for 145 galaxies. The LEGA-C MZR matches the
normalization of the $z\sim0.8$ DEEP2 MZR where they overlap, so we combine the
two to create an MZR spanning from 9.3 to 11.1 log$(M_\star/\textrm{M}_\odot)$.
The LEGA-C+DEEP2 MZR at $z\sim0.7$ is offset to slightly lower metallicities
(0.05-0.13 dex) than the $z\sim0$ MZR, but it otherwise mirrors the established
power law rise at low/intermediate stellar masses and asymptotic flattening at
high stellar masses. We compare the LEGA-C+DEEP2 MZR to the MZR from two
cosmological simulations (IllustrisTNG and SIMBA), which predict qualitatively
different metallicity trends for high-mass galaxies. This comparison highlights
that our extended MZR provides a crucial observational constraint for galaxy
evolution models in a mass regime where the MZR is very sensitive to choices
about the implementation of AGN feedback.
