Does Plesk support Amazon S3 compatible Backblaze B2 storage?
It doesn't have fully implemented support as there are side effects with incorrect backup removal.
Vote in the following UserVoice request to be implemented: https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/31897747-backblaze-b2-backups
Note: according to the documentation:
A number of services offering remote storage use APIs compatible with Amazon S3. We call them S3-compatible. Of those, Plesk explicitly supports DigitalOcean, Minio, and OpenStack.
You can try configuring any other S3-compatible storage in Plesk, but there is no guarantee that it will work as expected.