My friend sent me video proof of them getting the Regain Feather on retail without any hacks or mods. They didn't want to be identified, but since I thought it was worth sharing (I've never seen that before), they let me upload to youtube for them if I blurred their personal info.
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There's basically two tricks to it. One has been mentioned before: From ground level, there's a ramp with a misleading boulder that doesn't quite fully block running up the ramp, so you can actually get up there if you don't run directly into the boulder and get pushed off. There's probably only enough time to give maybe 2 or 3 attempts at running past the boulder. And there's a Luminant not too far from it.
The other trick is timing the two miasmas past the boulder. Apparently they run off the same timer, which stays open for a random amount of time between approximately 15~21 seconds. It takes 14~17 seconds with max move speed to go from the first miasma to the second miasma. So if you manage to time the first miasma just as it's coming down, you will usually have just enough time to run through the second miasma too. Because the fade timer on the Mimeo Stone is so short, the viable window of time gives you just a few seconds leeway if you arrive too early (before the first miasma comes down). If you arrive too late (after it comes down), it's much more likely you will run into the second miasma still being up.
Anyway, the video does show that this is indeed possible on retail without hacks. However, given that the route in the video already took ~66 seconds from start to finish, it seems unlikely that anyone would be able to do it by taking the normal route or without having max move speed.
