For what it's worth, I think of a journal as append only by definition and that's what SpiderOak does. Unless you have millions of very small files, the journal is going to be tiny relative to the backup content so this is fine.

So the server doesn't have a concept of "an existing file was updated" vs "a new file was uploaded" etc. The server only knows "new blocks have arrived." All the "smarts" are on the client.

In general operation, only new journal entries and new blocks are added. The only time blocks are removed is when the user intentionally chooses to remove data (we call that operation "purge") Intentional purges can also reduce the total size of the journal, and this the only operation that does so.

Most backup software removes previous versions and deleted files after 30 days, but SpiderOak keeps these indefinitely by default, to allow for for point in time recovery, restore from ransom ware infections, mistakes you don't catch right away, etc. You can set a different retention policy if you prefer.