According to John Romero, Doom speedrunner Zero Master has finally uncovered Doom 2's best-kept secret: a previously inaccessible sector found in the Industrial Zone map designed by Romero himself. You can see the moment Zero Master obtains the secret at around 3:04 in the video above, but I recommend starting at around 2:30 to get the full effect.
This secret has technically been known for some time, but Zero Master was the first to obtain it without using any cheats. As Zero Master explained on YouTube, "a secret can only be triggered if Doomguy's center is within the secret area and if he is on the same height as the secret sector." But as Romero explained on Twitter, you don't enter this secret sector when you touch it by normal means, "so you would never get inside the teleporter sector to trigger the secret." As a result, per the Doom wiki's Industrial Zone entry (opens in new tab), it was considered impossible to get this secret, "therefore the maximum secrets percentage one can ordinarily get on this map is 90 percent."
Zero Master found a way around this by luring a Pain Elemental and using the knockback from its attack to bump him into the secret. "This forces you down to the lowest floor the moving door is on, which puts you within the secret sector and on the same height, thus triggering the secret," Zero Master said.
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! #doomAugust 31, 2018
The Doom wiki's entry for the map has been updated following Zero Master's breakthrough. It now reads: "While the mechanism is not currently completely understood (it may be related to the phenomenon of sprites flickering across ledges or lifts), the resulting series of clipping operations will temporarily lower the player's z coordinate to the lowest floor underneath the player, causing the secret to be tallied."
In other words, after 24 years, Doom 2 speedrunners can now 100-percent the game without relying on cheats.