$ restic mount --repo /mnt/backupdrive/myrepo /mnt/backedup
$ echo /mnt/backedup/snapshots/*/path/to/file
And it would print which snapshots it is contained in.
If you don't know the path, I don't know how long `find -type f` takes so you might be right about that being inefficient. It is certainly not a use-case that I think restic ever had in mind.
I also don't know of "backup" software to solve this, it seems a bit out of scope for most of them (they are meant to have a backup copy of your disk, not be a file manager with history). You might have more luck with tools like rsync (or Toucan, from the good old days where I used Windows and Portable Apps), those can certainly create only new files and never delete deleted files, but then you typically don't get encryption and deduplication.
