A good deal of people in the online Greyhawk community obviously got their start or continue to play older editions of AD&D or 3.5E. This is perfectly understandable given Greyhawk's continuous publication run through that era of the 80's and 2000's. Most of the classic mods we all grew up playing that are being adapted and even expanded upon today, came out in the 80's. 3.5E on the other hand benefited from the ease of the d20 system, Living Greyhawk's convention model and the rise of Paizo publishing's Dragon/Dungeon run of Adventure Paths. Thanks to the OGL, Paizo's success was so good they drew off a huge swathe of Greyhawk's fanbase to join their new Pathfinder system and setting
4E was a definite stumbling block and it's now regarded in hindsight as a terrible edition. I mean look at the 0% polling. Even OD&D garnered 3% and it came out before the internet! Mind you I really, REALLY tried to promote Greyhawk adventures written the 4E format when they came out for eDungeon or eDragon, but general distaste for the rules and WotC's online subscription model was a huge barrier. This was also despite the popularity of Chris Perkins doing most of these mods, it's amazing 4E Greyhawk fell on deaf ears.
In reaction to the failure of 4E, 5E paid service to the disaffected loyal D&D fans and has been catering to the nostalgic crowd ever since, and to popular acclaim (Forgotten Realms be damned). Interestingly 5E still utilizes elements of 4E that were reviled. It's really more a stealth 4.5E than modern AD&D. Regardless, 1/3 of Greyhawk fans seem to love the current system and all the while without any sort of setting support!
Lastly, 5% of the readers chose other which is great as well. Greyhawk is an easy medieval fantasy world that has always been easy to adapt to any game system you prefer be it Castles & Crusades, Savage Worlds or Dungeon World. Alot of published Greyhawk sorucebooks like the Living Greyhawk Gazetteer or Iuz the Evil are essentially editionless material. Outside of fansites like Canonfire or Dragonsfoot that sort of book is never going to happen again folks,
And what system do I run my Greyhawk campaigns in? Sure I used to use the old OD&D and 1e/2e systems for a long while, but now they are only used for reference. Of course I then ran 3.5E to death. The Epic expansion nearly broke my spirit, but I still loved the ease of adapting stats to 3.5E and later Pathfinder plus I didn't have to teach rules to my players so we got down to the story faster. My Sea Princes campaigns still sometimes use Pathfinder/3.5 hybrid rules. I skipped 4E right out cause yes, I didn't feel like learning a new system. It was fun as a skirmish game and that's it. 5E on the other hand, I jumped right in after having fun playtersting it at GenCon and after my friend successfully ran the Tyranny of Dragons adventure. So far the simplicity of the current system and the lack of splatbooks to burden us down has been nice. Let's hope 6E doesn't come before 5E Greyhawk.