Galaxy clusters trace the evolution of the initial density perturbations which seeded cosmic structure in our Universe, making them powerful tools to constrain fundamental cosmological parameters such as dark energy and the fractional energy density of matter. The next-generation instrument on the South Pole Telescope, SPT-3G, has enabled the production of the deepest, mm-wave detected galaxy cluster sample to date over the 100 deg² SPT-Deep survey. With 442 optically confirmed clusters at a median redshift of 0.74 and a median mass of 1.7 x 10¹⁴ M⊙, this cluster sample provides the first look into a new regime of cluster populations accessible for cluster cosmology. One key astrophysical contaminant for mm-wave cluster cosmology is emission from star-forming galaxies as it is spatially correlated with the negative thermal Sunyaev–Zel’dovich signal used to detect galaxy clusters; this can alter the cluster selection function—particularly for low-mass, high-redshift systems—and potentially bias recovered cosmological parameters. For the first time, the magnitude of this contamination at high-redshift has been quantified with combined data from SPT and Herschel SPIRE, and we find that the tSZ signal is mitigated by dust at high-redshift (z > 1) by 4% at 95GHz, and 18% at 150GHz. However, we find that SPT’s 220GHz band provides sufficient leverage on dust emission to prevent this contamination from significantly impacting the cluster selection function over the mass and redshift range probed in this work. This marks an important step for validating the cluster selection function for the full forthcoming SPT-3G 1500d cluster sample that will be used for the next-generation of cluster cosmology measurements.