As has been said before, no learning or knowledge can ever be completely lost.

"I, too, had understood that such knowledge was lost long ago."

In particular the knowledge of the most traditional and simple method had been lost.

This knowledge is then lost completely if one or two staff members leave.

That's the main reason why knowledge of this system was completely lost.

That knowledge had simply been lost because it was no longer needed.

More likely the knowledge of their origins was simply lost over the ages.

What talent, knowledge and experience is lost because of this!

The knowledge and skills are lost, but what they did can be roughly understood.

What I regret is that so much knowledge has been lost.