Two-dimensional electron gases are an essential building block of today's technology and attract broad interest in the context of material science or nanoengineering. Nowadays, 2DEGs are employed for computing, metrology, spin to charge conversion, and optoelectronics. Usually, these low-dimensional channels spontaneously form or can be electrostatically induced at the interface of polar materials. The polar scattering can limit the electron mobility if electrons attain a temperature comparable to the LO phonon frequency. The remote LO coupling is of major concern in gate insulators with a high relative dielectric constant. In order to uncover the relevance of remote phonon coupling, we perform here the ultrafast spectroscopy of an accumulation layer. The quasi-2DEG is obtained by evaporating cesium (Cs) atoms on the surface of indium selenide (InSe) at low temperature and in ultrahigh vacuum conditions. This doping method simulates, with good accuracy, the electrostatic gating and can be easily implemented in our experiment. The choice of polar material is motivated by the fact that InSe is one of the best van der Waals structures for the fabrication of FET devices. It has an electronic gap comparable to silicon, small effective mass, layered structure, and carrier mobility higher than transition metal dichalcogenides. The electronic states and distribution function of hot electrons in the accumulation layer is directly monitored by time- and angle resolved photoelectron spectroscopy (tr-ARPES). We find that, hot electrons in quasi-2DEGs display a remote coupling to polar optical phonons persisting up to high electronic density. The accurate modeling of such interaction should include the wavefunctions of confined 2D electrons, dynamical screening effects, surface plasmons polaritons, and interface phonons. Nonetheless, the static screening of bulk phonons by 3D electrons can quantitatively reproduce the experimental cooling rate. This finding highlights that electrons in the accumulation layers or 2D conductors at the interface with a polar medium experience 3D dissipation channels. The outcome is of high relevance for the carriers' mobility in FET devices with high dielectric gates, van der Waals heterostructures, and 2DEGs at the interface between oxides.