Using the two types of the finite-temperature (T) local-field corrections, namely, the Singwi–Tosi–Land–Sjölander (STLS) and Hubbard (H) approximations, we investigate the exchange–correlation (XC) and T effects on plasmons (PLs) in strongly-correlated two-dimensional electron systems with ultralow density. In a density-parameter range of and in a Fermi-T normalized T range of 0.5 ≤ T/TF ≤ 8.4, we analyse those PL dispersions, which were observed extensively from single quantum wells by angle-resolved Raman spectroscopy, but which have not been analysed theoretically so far. Even in our strongly correlated electron systems, the size of the XC holes can be well normalized by the inverse Fermi wave number . Close comparison with the experiment shows that the STLS approximation can be expected to give a quantitative description of the PLs in a smaller wave-number (q) range of q kF, but that it overestimates the XC effect in a larger q range of q kF. The H approximation is insufficient to explain the remarkable XC effects. The conspicuous T dependence in the PL dispersion can be ascribed largely to the T dependence of the constituent electronic transitions of the PLs through the Fermi–Dirac distribution function.