Good to know, thanks! This is a "Onedrive for business" on the "Office 365 A1" plan, and I'm told by the admin that there are no filesize limits they are aware of. I haven't tried to upload a large file via the Onedrive client, which may well succeed. Since it's a education related account, my only option to access it via rclone is to use WebDAV. Perhaps there is a WebDAV-related limit that no one is aware of.
From my experimenting using rclone's WebDAV (v1.52.0):
1GB file: uploads fine
1.9GB file: uploads fine
2GB file: Starts upload, but I keep getting an html page output saying: "failed to copy. This is a temporary issue, so try again in a few minutes."
3GB file: does not even begin to upload; gives "413 Request Entity Too Large" upfront
So 1.9GB seems like the limit. Time to use the chunking backend.