Can I simply delete old directories from the official “Steinberg Download Assistant” download-directory? For Cubase alone, these downloads take up 6GB of storage space. Or does the manual deletion possibly mess up the Download Assistant? Do these still have any use after the download or installation? Here are some examples:
Thank you for feedback from people who are familiar with it. Please no suggestions like: outsource content 
You can actually delete the entire Steinberg “OSX” folder completely after you’ve downloaded/installed software and other content/libraries. On macOS, your download/component download/installation history is maintained through metadata and not via the existence of the downloaded files themselves - Meaning, when you download a component and delete the OSX folders, it still registers as “installed.”
So not just “old directories,” but “all of them” if you choose to.
As @Thor.HOG wrote you can delete the files after they have been installed, but I always recommend to keep at least 2 or 3 previous versions, just in case the latest release has a problem and you want to go back.
The Download Assistant allows you to specify a download location, other than the default. I have it changed to an external SSD drive with 4 TB space, so I don’t occupy space on my system drive, but can always delete the latest release and pick any of the earlier ones.
Further, while we’re on the subject, the detailed json “installation state” metadata is stored in:
~/Library/Application Support/Steinberg/Download Assistant/SDA_downloads.json
This shows the state of installed components. Uninstalled components won’t be in the json file, and thus, show us uninstalled in the SDA. While probably unsupported, if you did a full library copy to another machine and re-registered the libraries, you could just copy this file over to the new machine and it would show everything as installed again.
Probably just me, but if I know I’ve registered a library, I like to see that reflected in the the SDA even if I didn’t actually download it. This works for me, but probably isn’t something the user should be mucking about with in general. But since registrations from SLM don’t update SDA, (and probably shouldn’t) I just wanted to point that out for the more daring folks 
That sounds good. However, I do not see this file in this directory:
I have two HALion VSTi, which are installed but not shown as installed in the SDA: “Materials Metal & Ice” + “Materials Wood & Water”. Is there any way to add them again without downloading and installing them again?
That screenshot shows you in the root /Library, not the user’s home directory library indicated by the “~” before ~/Library. The “~” is a shortcut to the /Users/[username] folder. SDA metadata is stored per user, not per system 
The general “Materials” libraries are only about a gig each, so I would just install them again. If you have a copy of the json file from another installation, you could copy just those entries and edit your local json file, but I wouldn’t do that.
IMO, interacting with the SDA_downloads.json file really only makes sense if you’ve duplicated/mirrored a configuration on another system. Trying to manually edit state for individual libraries is a process of diminishing returns to me….
This can be really a dangerous thing, because the structure of the file will be checked for validity and just leaving one curly brace, or a little single quote out can render the file invalid. So yes, don’t do that.
If you really want to do this I recommend to use a tool like Visual Studio Code for editing, because that shows the structure in colors and if there is something invalid you will lots of red and a error message saying what is wrong.
Indeed. Which is why I pointed out it was (probably) unsupported, and that in general users shouldn’t muck about with it - the “unless you know what you are doing” bit was inferred, but I appreciate the more explicit warning!
