Backup to Yandex Object Storage via Duplicati are Yandex directions that use S3 to their compatibility API.
Possibly that will work for now. The more code that needs writing, the slower this will possibly get done…
Yandex disk support is a similar request, where at the time Yandex WebDAV interface did the job perfectly.
Disk WebDAV API only talks about Yandex OAuth authentication which I don’t think Duplicati WebDAV has. Still, it might be worth testing if the above previously tested WebDAV username/password interface works. That would also avoid the need to study legal pages and fill out application forms possibly only in Russian.
REST API is another API needing OAuth. I don’t know how similar it is to the other REST APIs in Duplicati. Depending on that study, this might need the most work, WebDAV less so, and S3 is possibly already set.
This will probably take awhile to sort out. Perhaps technical comments from other people will accumulate.