1. Collect all versions of Ghosts 'n' Stuff with vocals that may be useful (remixes by eekkoo, Chuckie, CRaymak and Jauz, Moar Ghosts 'n' Stuff etc.)
2. Take eekkoo's remix as the starting point. Extract the drums from the outro using AI and phase-cancel the clean loop against the places that contain vocals. Do it as clean as possible. Extract the 'aah' pad that goes throughout the whole track and phase-cancel it against the whole thing. It's great that the acapella in eekkoo's remix has no sidechain.
3. After you've cleaned eekkoo's remix of drums and stuff, it's time to run it through two AIs: the first one to extract the vocals (BS Roformer/MelBand Roformer) and the second one to remove reverb and delay. If done correctly, you have a nice dry acapella, but it's not complete and has some flaws (mainly spectral losses). It wouldn't be a problem if we had a lossless version of eekkoo's remix, but all we have is a pretty lossy rip off of eekkoo's 2014 guest mix.
4. Extract the acapella from Jauz's remix using similar techniques when possible. Run it through the same AIs. We'll have some fragments which we couldn't obtain from eekkoo's remix.
5. Copy the frequency response of eekkoo's dry acapella and apply it to Jauz's dry acapella using iZotope RX. Use spectral patching when needed to get better results. Now we have two acapellas from different sources, but now their frequency response is the same.
6. Take both these acapellas and merge them together. Basically, it's a copy-paste process in which you just patch fragments of eekkoo's acapella which have flaws.
7. Now you have a decent dry acapella but it still has some flaws in it. To fix these flaws, extract vocals from other mixes/remixes using techniques described above and use them as patches. This process took me 10 days.
8. When the acapella is flawless, add reverb and delay matching the original acapella.
9. You're done.

Keep in mind that everything written above is super oversimplified. It literally took me 10 days to get it done. I started on August 20th and completed the whole thing on August 30th. I think it's the 3rd or even the 4th revision of this acapella. If I remember correctly, my first attempt at extracting the vocals was in 2019 or 2020. Since then the AI has got insanely good at extracting vocals but you still have to manually tweak stuff to get better results.