The newff function did indeed exist (in the Neural Network Toolbox).
According to this answer it became obsolete in 2010, although it remained available for some time after that. This is typical for Matlab: most obsolete functions will stay usable for several years to give people time to implement the replacement.
I can't find any mention in the release notes about this function. The online release notes go back 5 years, so it was removed prior to R2017b.
It appears to still exist in the nnet Octave package, but you should find the replacement function, which seems to be feedforwardnet.
