I do think worldbuilding should be with canon characters as default, but I don't think OC's are opt-in. They're the other main option to actually have characters in your fic if you're not using canon characters and you don't want to write the "records of grain harvests for the past 5 years and how the wars affected gross output" kinds of worldbuilding. If there's a time period that's only ever referenced in canon that none of the canon characters experienced, or a character that's named, but literally nothing else is known about them, OCs are a good way to write worldbuilding in those situations.
I don't think requesters of worldbuilding should be allowed to direct their writer and ask for "Fic set during this canon time period with my OC, Ebony Dark'ness Dementia Raven Way, who's Ravenclaw's descendant and she has a 12in ebony wand with a Thestral heartstring core and this is her outfit," but ODAO is still a thing and they can't even for non world-building requests.
Re: Worldbuilding is go!
I do think worldbuilding should be with canon characters as default, but I don't think OC's are opt-in. They're the other main option to actually have characters in your fic if you're not using canon characters and you don't want to write the "records of grain harvests for the past 5 years and how the wars affected gross output" kinds of worldbuilding. If there's a time period that's only ever referenced in canon that none of the canon characters experienced, or a character that's named, but literally nothing else is known about them, OCs are a good way to write worldbuilding in those situations.
I don't think requesters of worldbuilding should be allowed to direct their writer and ask for "Fic set during this canon time period with my OC, Ebony Dark'ness Dementia Raven Way, who's Ravenclaw's descendant and she has a 12in ebony wand with a Thestral heartstring core and this is her outfit," but ODAO is still a thing and they can't even for non world-building requests.