Temporal evolution of the Na I D λλ5890, 5896 EW in the spectra of AT 2021biy. The Na I D measurements are performed through the Markov chain Monte Carlo (MCMC) methods using the EMCEE PYTHON package (Foreman-Mackey et al. 2013). Spectra are trimmed to the region surrounding the doublet and are fit with a double Gaussian subtracted from a linear continuum. The MCMC implementation initialises walkers for each parameter of the model and iteratively perturbs each parameter, checking at each step whether the fit to the data has improved. Upper limits are marked by empty symbols with downward-pointing arrows.