  Effective yields, $y_{\rm eff}$, are defined by fundamental galaxy properties
(i.e., stellar mass -$M_{\star}$-, gas mass -$M_{\rm gas}$- and gas-phase
metallicity). For a closed-box model, $y_{\rm eff}$ is constant and equivalent
to the mass in metals returned to the gas per unit mass locked in long-lived
stars. Deviations from such behaviour have been often considered observational
signatures of past feedback events. By analysing EAGLE simulations with
different feedback models, we evaluate the impact of supernovae (SN) and active
galactic nuclei (AGN) feedback on $y_{\rm eff}$ at redshift $z=0$. When
removing supermassive black holes (BH) and, hence, AGN effects, in simulations,
galaxies are located around a plane in the $M_{\star} - M_{\rm gas} - {\rm
O/H}$ parameter space (being O/H a proxy for gas metallicity, as usual), with
such a plane roughly describing a surface of constant $y_{\rm eff}$. As the
ratio between BH mass and $M_{\star}$ increases, galaxies deviate from that
plane towards lower $y_{\rm eff}$ as a consequence of AGN feedback. For
galaxies not strongly affected by AGN feedback, a stronger SN feedback
efficiency generates deviations towards lower $y_{\rm eff}$, while galaxies
move towards the opposite side of the plane (i.e., towards higher values of
$y_{\rm eff}$) as SN feedback becomes weaker. Star-forming galaxies observed in
the Local Universe are located around a similar 3D plane. Our results suggest
that the features of the scatter around the observed plane are related to the
different feedback histories of galaxies, which might be traced by $y_{\rm
eff}$.
