I deleted the wrong snapshot from back-in-time backup. Is there a way to retrieve it?

Depends on the file system type the snapshot was stored on.

If it was ext4...delete is delete. But likely still exists if you haven't done any big writes to the drive.

You can try photorec which is part of testdisk to scour the partition for files by type, or data carvers like foremost that will grab everything but the names, files found are typically named by the offset sector of the drive where it starts or something. Since you have other snapshots you should be able to figure out it's size (more or less), might be easy to figure out which file it would be if found. Sleuthkit is a forensics tool, might be able to get list of files by size, haven't used it in a long time.


Typically it's best to do this from a live CD in read only mode and send results to another physical drive for best performance.

If your FS was EXT(3 or 4), I would suggest to try R-Linux (https://www.r-studio.com/free-linux-recovery/) It's not open source, but it is free. The free version works only on EXT FS.
This gives you a very good chance to recreate the whole directory and all the file names. Although I normally prefer Photorec, this might be the rare occasion wher R-Linux gives you better usable results.
Very easy GUI.
Do not install on the disk you want to carve of course...!

No.

It reportedly uses hardlinks for (so called) backups. Once they're gone, they're gone. New files (in the backup) may be retrievable as per above, but that is all. Why do you care ?. What is potentially missing that you care about ?.