There are at least two defaults that have been changed: pseudopotential/basis set (FHI/DZP in 2016.4) and electron temperature (300K in 2016.4) .
In 2018, 'Electron temperature' parameter is called 'Broadening' (which is 1000 K in 2018) to reflect the fact that this is not a real electron temperature, but rather a numerical parameter to make SCF convergence smoother with respect to the number of k-points.
In 2018, the default pseudopotential/basis set is pseudoDojo/Medium.