  The ratios of strong rest-frame optical emission lines are the dominant
indicator of metallicities in high-redshift galaxies. Since typical strong-line
based metallicity indicators are calibrated on auroral lines at $z=0$, their
applicability for galaxies in the distant Universe is unclear. In this paper,
we make use of mock emission line data from cosmological simulations to
investigate the calibration of rest-frame optical emission lines as metallicity
indicators at high redshift. Our model, which couples the SIMBA cosmological
galaxy formation simulation with cloudy photoionization calculations, includes
contributions from HII regions, post-AGB stars and Diffuse Ionized Gas (DIG).
We find mild redshift evolution in the 12 indicators that we study, which
implies that the dominant physical properties that evolve in our simulations do
have a discernible impact on the metallicity calibrations at high redshifts.
When comparing our calibrations with high redshift auroral line observations
from James Webb Space Telescope we find a slight offset between our model
results and the observations and find that a higher ionization parameter at
high redshifts can be one of the possible explanations. We explore the physics
that drives the shapes of strong-line metallicity relationships and propose
calibrations for hitherto unexplored low-metallicity regimes. Finally, we study
the contribution of DIG to total line fluxes. We find that the contribution of
DIG increases with metallicity at z $\sim$ 0 for singly ionized oxygen and
sulfur lines and can be as high as 70% making it crucial to include their
contribution when modeling nebular emission.
