A Microsoft file hosting and synchronization service.
You're correct that OneDrive for Mac now uses Apple's File Provider framework, with synchronized files. However, this does not necessarily mean that the older OneDrive.noindex folder is no longer used or can safely be deleted. Microsoft has previously identified ~/Library/Group Containers/UBF8T346G9.OneDriveSyncClientSuite/OneDrive.noindex as a OneDrive cache location and advised against manually removing it. The fact that your new test document appears in CloudStorage but not in OneDrive.noindex is useful information, but it does not conclusively establish that the 28 GB folder is abandoned. Similarly, the 261 GB .Dbfs.Dbfs_Personal.noindex folder may be associated with OneDrive's internal storage. Its reported size does not necessarily represent an additional 261 GB of physical disk usage.
To determine whether these folders are still associated with your current OneDrive installation, could you confirm which version of OneDrive you are running (select the OneDrive cloud icon in the macOS menu bar > Settings > About) and whether you installed it from the Mac App Store or directly from Microsoft? Also, does OneDrive currently report that all your files are synced, and are you trying to recover disk space or simply remove folders you believe are obsolete? If possible, please share a screenshot showing the two folders and their sizes, along with System Settings > General > Storage so we can compare the reported folder sizes with actual disk usage. If your objective is to recover space, Microsoft's supported Free Up Space option for locally downloaded OneDrive files would be a safer starting point than manually deleting internal cache folders.