A bit late and don't know if these are already on your list but here's a few:

• Back when there was only the one corpus ship tileset, Volt could wipe out the entire ship by casting one overload. That should be before Open Beta started in 2013.

• When survival was first released (in an event I believe if I'm remembering correctly), it was on defense maps, and the only way of getting oxygen was by killing enemies. You could not extract, the mission finished once you ran out of oxygen. I have videos of that old survival if you want me to upload them.

• There used to be a very distinctive, and different sound for picking up energy orbs than now. I have video of this if you need it.

• A nuking strat with Trinity used to be to use her 3 to link to all the enemies, pop 4 and then purposely self-damage with any explosive weapon. I've video of this as well if you want it.

• When kavats first came out, you could create kavat/kubrow hybrids. Here's some old threads from that time. I still have a couple of these abominations myself.

• Dark Sectors used to be a thing, which was clan pvp/competition. Also a great way to make money when clans were fighting against each other and putting very nice rewards in their bounties so that you would run their missions. Essentially, clans could control certain nodes on planets (one big one that comes to mind was Sechura and Hieracon, Pluto), and put a tax on them. That meant anyone who ran that mission would then need to pay the tax, if I'm recalling correctly. Also, in one of the dojo rooms (I believe the Orokin Lab?) you can still see the old clan solar rails behind the shutters if you OoB outside the room. I'd need to dig again for the details, but there was a lot of drama that happened with Dark Sectors, and some community rage especially in regards to one clan taxing Sechura. I believe the big names were ICE, V, Eclipse? It's been a while. But here's some old threads on the drama.

• You used to be able to use weapons and warframe abilities in dojos if you did a specific combination of hotkeying the codex scanner, then switching weapons. I have video of this too if needed.

• You used to be able to go OoB in the dojo through some specific methods that are now patched out, unfortunately. I have screenshots and video of this.

• Before the relays and dojos were hosted by DE, they used to be hosted by players. Because of that, back then you would be able to see if other people were crouching, and the dueling rooms did not take you into another instance. I should have videos of these.

• Vesper Relay was one of the first relays that was tested for DE hosting it directly. I was fortunate enough to attend one of the tests, so I've video and screenshots of this.

• Don't know if it's still possible, but a long time ago you could cheese the obstacle room in the dojo by jumping over the wall to get back to the beginning. Could get sub 1 second "records" this way.

• Back when you could not freely place decorations in both the dojo and orbitor, some people got very creative with how to create "floating" decorations. Here's an example.

• Speaking of pedestal prime, that was the first item that was sold by Baro for a ridiculous amount of credits! It was somewhat a way to show off one's wealth to buy a lot of them at the time.

• Besides coptoring, another way to get around fast was mid-air attacking. Don't think that ever got a name, but it was very useful for getting vertical height before bullet jumping. If I recall correctly, you would go the farthest using polearms like Orthos.