Beat Normal without upgrades. I died a couple times to the stage 5 boss which I find to be the hardest boss. The third form of the 2-5 fight is particularly long and tedious without upgrades. Didn't go into the upgrade menu at all during stages just in case. I chose not to get any of the recordings as I was curious if they were bonus content or if they're required for the secret ending
where you can revive Gehnie.
Doing a no upgrade run changes two major events, namely the dialogue at the end of stage 2-2 where Gehnie comments that the enemy's power is tied to Irina's awesome dodging skills (instead of the ship itself being powered up) and during the final ending sequence where Gehnie comments that it was amazing that you were able to defeat the Kharlal Empire without ever upgrading. I'm not sure if buying upgrades locking you out or if equipping them technically does? Energy Tanks actually max out at 99 and you stop gaining them from there.
Sadly, the ending where Gehnie is revived was NOT triggered. I'm not sure what the conditions are, but I have a few more things to test:
1) No upgrades, all recordings watched, maybe need to no-miss as well? I'd be surprised if this required a specific difficulty mode given how tedious no upgrades gets in the loop. There's a few tanky enemies that'd make getting all the recordings tricky!
2) Ridiculous Difficulty with all recordings watched before the ending, hopefully doesn't require a no-miss. (I could've sworn I'd done this before but maybe I skipped though the plot at the end accidentally? On my no-miss run of Ridiculous I actually missed the very last recording so that's no help.)
Possibly the dialogue in event S21 can't actually be triggered in-game. Maybe it's leftover data or something they were going to use for a secret ending event or something but decided to remove from the final build or who knows what? It's possible... No other footage on Youtube or Nicovideo exists that shows an ending event taking place after the normal ending, and nowhere does anything suggest what the trigger requirements might be.
edit: beat Hard mode without upgrades and all recordings... or at least I tried to, but apparently pausing the game to listen to the recordings in the middle of each level breaks the no upgrades thing. To trigger the no upgrades dialogue
you must not go into the menu at any point in the middle of a level, even if you never touch the upgrade section. I think you can safely still listen to recordings in between levels after each boss (but it means you can't listen to the 2-6 recording and trigger the "no upgrades" dialogue after the final boss). No secret scene yet.
No upgrades gets really tedious due to how long bosses take to kill so I'm going to try Ridiculous with all recordings and aim for a no-miss again if possible and hope that something like that will be the key.
edit DEJA VU: Beat the game on Normal without upgrades, all recordings obtained and watched only in-between levels, without dying. The special dialogue triggered in 2-2 and 2-6 as expected but nothing after the ending.
THIS TIME THE DEFINITELY LAST EDIT:
Beat the game again on Ridiculous, all recordings watched before the final boss, used powerups. No secret scene. If scene S21 is supposed to be accessible in game, that means the requirements are ultra strict and therefore silly such as:
1) You need to no-miss Ridiculous while getting all recordings. This is much harder than it sounds as the recordings in Stage 6 are both positioned by turrets that can kill you very easily as they pop up.
2) You need to beat Ridiculous without upgrading and possibly some other combination of requirements? I haven't done this yet as it'd be silly and I'd be sad if a "true ending" scene or something were locked away behind this.
3) You need to beat Easy mode...? I've actually not really touched Easy mode, I'm not sure why an additional story scene like this would be exclusive to it, which strikes me as extremely odd in the unlikely chance this is the case.
Or, and this is the possibility I'm leaning towards more given my testing has proven fruitless:
3) Scene S21 may not actually be accessible. It may be leftover text that they decided to not use and never made accessible in-game. Making Gehnie come back alive as Gehnie Ver 2 and then be sent back in time by Irina suggests that the Kharlal Empire is somehow still around in the future, contradicting the recording where future Irina sends Gehnie back initially. The whole goal of the ending is achieving a future where the Kharlal Empire and thus Izumo no longer exist, so Gehnie would never need to be created and sent back.
I'm kinda burned out on Drainus at the moment as I've been playing it several times trying to see if the text in scene S21 is accessible in-game to no avail. So, if anyone else uncovers it, let me know, but I think I'm done!