+1 to rvisima's comment:
The entire purpose of keeping my OneDrive files on an external volume is so that they are never, ever, ever on my home drive. I do not consent to OneDrive ever touching any files on my home drive, period.
Is it really the case that such external drive-only experience is still not possible, and won't be possible in the foreseeable future?
Tresorit can do it. Dropbox can do it. Does this mean that these services will eventually also need to drop support to external drive-only capabilities?
What's the expectation for OneDrive to ever being able to handle the external drive-only use-case again? Aren't there alternative solutions you could pursue so you could give us anything more specific than "in a future version of macOS" (e.g. write your own kernel module)?
Thank you for listening to your customers and taking steps in the right direction, please continue.