It seems Duplicati already supports Keystone v3, so this is only a matter of adding a new endpoint.
FYI, here is the email I received:
Dear customer, A few weeks ago, we notified you by email that we are discontinuing v2 of the Keystone API on 24th March 2020. All users of the Keystone API must now use version 3, and make the changes required to do so. Our last analysis shows that you are still using v2 of the API. With most updates of this nature, you just need to modify the endpoint and the API version, but this change requires specific attention, which must be executed before 24th March 2020. Please update your tools or applications accordingly. You can find more information on our status page. Thank you once again for choosing OVHcloud. The Public Cloud Team  https://ovh.slgnt.eu/optiext/optiextension.dll?ID=dStdoW+qPGF+x+s02CEdQ_I66qw4G_Z_rqsNrzqCkOZiWQwdnmWGsZerWXZIQoWQpcyyq3Su3addd_  http://travaux.ovh.net/?do=details&id=42179