They fixed that in pgbackrest a while ago: https://pgbackrest.org/user-guide.html#backup/block

It was a major pain point for my backups for years.

Does that work with S3, etc.? I don't remember them allowing partial file uploads.

I believe so, because it is done in conjunction with their file bundling feature and doesn't rely on support from the blob storage backend.

They create a new file with the diffs of a bundle of Postgres files, and upload that to blob storage.