That happened with the Patents Act 1953, where the phrase "or process" was added by Congress.
According a legal law professor this was also when they removed the requirement for a working model, but I suspect that this was actually the date when the process to deposit a model was removed. There is plenty of models being sent after 1880 to USPTO (according to museums who write about their collections), so clearly it didn't all stop 1880. Also according to internal policy documents from inside USPTO that dates to early 1960, "the only thing you need to bring us a working model of is anything you claim that reverses entropy", which again highlight a change in attitude during that time in history.
That said, you are right that the official date when the requirement ended was 1880.
According a legal law professor this was also when they removed the requirement for a working model, but I suspect that this was actually the date when the process to deposit a model was removed. There is plenty of models being sent after 1880 to USPTO (according to museums who write about their collections), so clearly it didn't all stop 1880. Also according to internal policy documents from inside USPTO that dates to early 1960, "the only thing you need to bring us a working model of is anything you claim that reverses entropy", which again highlight a change in attitude during that time in history.
That said, you are right that the official date when the requirement ended was 1880.