We have investigated the effects of oxygen deficiency (δ) on the transition temperature (Tc) of (Sm, Er)Ba2Cu3O7−δ superconductors by incorporating the effects of the two dimensional (2D) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO2 plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation ofTc withδ in Y Ba2Cu3O7−δ system. The coupling strength (λ), the screening parameter (μ*) and the two dimensional acoustic phonon (plasmon) energyħω−(ω+) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium cuprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er)Ba2Cu3O7−δ superconductors.