Let's break down the riddle first. "You have found a pedestal." People have come to think that the middle platform can be considered a pedestal but I in fact believe that the whole room is the pedestal. "The chest was there at some point." Obviously the key word is WAS, because we cannot see it. "It lies between future and past..." I don't know but the way that's put together automatically means to me that past and future are physical objects and do does everybody because they find them to be the two gates. Maybe I am wrong but I have never heard them referred as past and future but them again I haven't really read the Vault's grimoire. But my point is that there are two different planets. And if the gates are future and past then we kinda mixed them up. The left would be future and the right would be past. And then it would make sense because the riddle states "future and past" and most of us happen to read things from left to right which places the gates correctly in the riddle. "...but not in the present." Now this is where I think we got it backwards. If the gates are physical then the last line must definitely be literal. Meaning that it isn't in the present where we stand after but someWHEN else entirely. That's backed up by the next line. "Time is broken, but it can be fixed..." This tells us that the chest is lost in time. "...for people have mended it before and left without questioning, what changed." Now we all know that in the VoG time is broken due to Atheon being time's conflux itself. Meaning he is time in itself. And most of us have killed Atheon and left without checking out the surrounding area. Which could be the last part of the riddle meaning directly that the chest is definitely around after the fight. Somewhere or somewhen.
Well I apologize if I confused you guys but I just wanted to break it down a little bit.
Edit 1: here's the link to the riddle:
https://www.bungie.net/en/Forum/Post/103722234/0/0
Edit 2: A poster commented stating that both portals are Venus. One is past and the other future. The left portal which we consider to be Mars is actually the future. A ruined place. And the right Venus. A habitable past. And then it fits the riddle perfectly!
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if I'm correct you can find plants growing in the present and no dead plants, so the future portal is actually venus.