It's currently scheduled for the 5.0 release of the Managed Backup console. That a couple releases away, so I'm trying to get some confirmation from the team if that's still the plan or if it might be out sooner. You can add B2 storage using the S3 - Compatible connector, but we have not certified synthetic backups with that configuration - I just wanted to mention that just in case you were going to try it (don't :-))

An update on this from the information I received from the team:

Synthetic full backups for Backblaze B2 (not S3 compatible) will be available when the new backup format is released in the 4.9 MBS version. At this time, we are not planning to support Backblaze B2 Synthetic Full backups using the B2 S3 API. I can't go into details as to why, but I believe it has to do with some limitations in the Backblaze's B2 S3 API implementation.

Thanks for the update. We don't have a lot of Image Backups as we have moved to Hyper-V virtuals for the bulk of our clients. But I just moved all of the Image/VHDx backups from Standard Backblaze to BB S3 so that I can use Cloudberry Explorer to manage it (vs the awful Backblaze Web Portal that takes several minutes to load the list of files).
On an semi-related note, how is the new Backup format going in the Standalone product?

What is the ETA for the 4.9 MBS release? We are also looking to implement synthetic fulls for our clients which are all stored in B2 (non S3 compatible)