Why wouldn't it be? An encrypted file with the appropriate cipher set is synonymous with noise. It is only the encrypted content plus the unlock key that makes the file contents accessible.

If it is irretrievable, and the crypted data looks like noise, I would argue that the content is gone.

Especially if quantum computing develops enough to break modern encryption.

Unless the encryption uses post-quantum crypto, in theory it’ll all be “un-shredded” if quantum computing becomes feasible. We’ll have bigger problems at that point, but still, it’d make you “unforgotten”.