I noticed that on old computers it's sometimes helpful to ad a boot flag to the boot partition. When selecting the flag function initially, there are quite a few options which could be used. So I'm wondering about the following ...

... is it possible to mark a partition as "non-existent" (invisible in the file manager), to prevent anyone from accidentally storing files on that partition? And if so, which flag name/descriptor would that be? In other words, that particular partition would only become visible again if it's unmarked by Gparted or some similar application.
Thank you.

You do not mount and write files to partitions, you write files to filesystems which are residing on partitions. If a filesystem is not mounted then it is invisible and cannot be used.

Oh, I see ..
Well, the idea came to me after a buddy of mine who's a Windows user should be that although he can only "see and use" his C:/ partition in his Windows Explorer, he can also see two additional partitions when he looks at the Windows Disk Manager. That's where you can see the system reserved as well es the system restore partitions which can't be seen anywhere else (without a terminal).
So my thinking was, if Microsoft can more or less hide partitions to prevent them from being used by accident, wouldn't it be possible to do the same thing in Linux? In Windows those two partitions exist, but can't be used. That's kinda what I was looking for.
Not possible?

You put your filesystem into /etc/fstab with 'noauto' option and it won't be mounted automatically and visible unless you mount it manually. See 'man fstab' and 'man mount', all details are there.

Set the flag to 'diag' ?

On msdos (MBR) formatted disks there is a "hidden" atttribute as well as the boot flag for partitions - Microsoft uses both, Linux ignores them. For gpt there is a hidden attribute defined (by Microsoft) for (Windows) basic data partitions. I imagine Linux will ignore it too, but haven't tested.

Thanks a bunch. Those comments were helpful. Have a great Sunday.