How can I delete a folder backed up via Time Machine to a .backupbundle disk image? When I try to delete the folder, I get the error message, "The operation can't be completed because backup items can't be modified."
I found a similar question, "How can I delete a file marked as backup item?" The accepted answer says that this is a bug that will be fixed in a coming update. Unfortunately, this answer was written in 2011 –– 11 years ago. I tried to comment on the answer, but I did not have enough reputation.
A little more precision regarding what you tried would be helpful. "when I Delete the folder". Which folder? Is that on the source disk, or is it in the backup? And how did you try to delete? Also what version of macOS and what format (HFS+ or APFS) is the backup disk image?
/Volumes/Time Machine Backups/Backups.backupdb/MacBook Pro (Retina)/2021-05-02-065359/Macintosh HD 1/Users/brianschack/Transmission Downloads
On the backup. I tried dragging the folder to the Trash in the Dock. macOS Big Sur 11.2.3. Mac OS Extended (Case-sensitive, Journaled).
The accepted answer on your linked questin is basically wrong. The highest voted answer is correct - you are trying to delete from a protected area. You cannot just delete Time Machine files from Finder.
@Ryccardo wrote the highest-voted answer to the question, "How can I delete a file marked as backup item?" He says, "[L]ocate the item in the 'in space' Time Machine view[.]" This is a backup of a computer that I traded in. When I go to the in-space Time Machine view on my current computer, it shows me the backup of my current computer –– not the computer that I traded in.
He also writes, "There exists a helper program (command-line binary) [...] which allows you to supply a command and argument to completely bypass the access restrictions." When I navigate to /System/Library/Extensions, I do not see TMSafetyNet.kext. In a different answer, @Evan Kroske writes, "It seems like the bypass program doesn't exist on Big Sur."