To be fair, Duncan knew it and technically there are realities with no survivors from that ritual… There are also realities where it appears your Gray Warden has been running around spiking people’s drinks with dark spawn blood because how the hell do we keep getting more wardens? It’s possible that the knowledge was really lost due to a few Wardens thinking “No, I won’t repeat that.”

The truth of the matter is, no one can bring themselves to put taint that many times in the modern games.