Why do DBs perform Delete operations online? Wouldn’t it be better to soft-delete (at the table-space level ) and then run scheduled task to clean up the table spaces?

Similar to git. When you “delete” files they are just removed from the tree. It isn’t until later that all refs and reflog references have expired , and gc is run, that the objects are actually removed.

If you read the linked article, it explains that this is exactly what they do.