Constraints on the total optical depth (top panel), and on the optical depth between z  =  15 and z  =  30 (bottom panel), when using 2015 PlanckTT+lowTEB data. Various reionization models and priors are used (labeled in each plot), in addition to marginalization over other ΛCDM parameters. Top panel: apparent preference for higher τ when using the PCA model as reported in Heinrich et al. (2017) was significantly impacted by the choice of prior. Bottom panel: evidence for early reionization was also partly impacted, although some preference remains. We consider the shaded green contour, which corresponds to a flat τ prior and optimal physicality region, as the most well-motivated constraint from this dataset.