Parmandur
Book-Friend, he/him
No, 5E has never had a "default" Setting and thisnis no different. The Forgotten Realms have usually been the example used, and where they ofteb set Adventures until 2020 (they have not usually used the FR for that since Icewind Dale, ao it is odd that people keep insisting all the Adventures are in the FR: they are not).I'm not sure I can find that anywhere, they seem pretty explicit in as recent as the DMG overview video from the other day that Greyhawk is the intended place to start, Perkins talking about not wanting to throw new players right into creating their own setting so Greyhawk is what they have to start with.
So again, if that isn't the default, I dont' know what is. Note, that's different from official. Any published setting is official.
The default assumption is homebrew, really, with the proposed multiverse frame Setting for DMs to pick what they like.