We have a content type that was incorrectly created on one of our site collections. 100 days ago, I deleted all instances of the content type in site collection and subsite libraries. I have since verified that the content type or parent content type does not exist in any library on the site collections or any subsites.
However, I still receive an error when attempting to delete the content type: "Another site or list is still using this content type. If you would still like to delete it, please remove the content type from all sites and lists and then try again."
Taking a second pass, I identified that a content type was using this as a parent content type, so I removed the content type and modified all files with that content type to a new content type with the same columns and a different parent content type. I've also cleared the recycling bin and second recycling bin.
I'm unsure how to identify where the content type is in use, as I've powershelled every library in the site and subsites, reviewing each Parent and Type. There doesn't seem to be a reliable way to identify content type usage in SharePoint Online.
Does anyone have advice on how to identify what reference continues to exist for the content type?
Not a bad idea - we don't have any retention policies defined, so there are no preservation hold libraries on the site collection or subsites.
That is a good point though - I wonder if the previous versions of a converted file being the content type might be preventing deletion. I'm unsure how to modify content types of past versions. That might be a venue to tackle.