The big picture: It's tough to determine what's more impressive here: the fact that people are still playing Doom II after nearly 24 years or the fact that it took this long for someone to figure out how to activate the game's last remaining secret.
It took nearly 24 years for someone to discover how to unlock Doom II’s last remaining secret without using cheats. The feat, recently shared on YouTube by user Zero Master, has since been confirmed by John Romero, one of the game’s original programmers.
As Zero Master explains in the video’s description, the secret location on map 15 (Industrial Zone) has long been known but was thought to be impossible to trigger naturally without cheats. Until recently, the DoomWiki for the secret read as follows:
“It is impossible to register secret #4, as the secret sector in question (sector 147) is located adjacent to the raised teleport pad and is only 16 units wide. This setup completely prevents the player from touching this sector's floor (as is required to trigger a secret), because upon crossing the sector's boundary they are immediately raised onto the pad and teleport away. Therefore, the maximum SECRETS percentage one can get on this map is 90%.”
So in other words, the secret can only be registered if Doomguy’s center is within the secret area and if he is on the same height of the secret sector, the latter of which was thought to be impossible because Doomguy is lifted up to the teleporter sector floor height when his center is on or over the secret sector.
Zero Master figured out you can lure a Pain Elemental into the area and have it spawn a Lost Soul on top of you, thus forcing you down and putting you within the secret sector. Romero explained it as follows on Twitter:
Secret explained: A secret teleporter is marked as a discoverable "secret", but when you touch it, you never enter the sector. So you would never get inside the teleporter sector to trigger the secret. The only way to trigger this secret is to be pushed by an enemy into it! #doom— (@romero) August 31, 2018
Zero Master said the chain of events that make it work simply do not happen by accident and there’s no reason to even suspect it should work, hence why nobody has done it before.