We’ve been hovering close to 100% for a while now but, with the staging-next merge a few days ago, it’s finally happened: https://r13y.com/ (Is NixOS Reproducible?)
If I’ve done everything correctly, the 21.05 ISO also passes the check on my machine, hoorray!
Feel free to verify everything yourself; getting @grahamc’s r13y script running is trivial (I’d recommend running it inside a Docker container, though remember to enable sandboxing!)
Next goal: 100% reproducible GNOME ISO 
Here is some general information what this is about and why it matters: https://reproducible-builds.org/
Here are comments to this announcement: Nixos-unstable’s ISO_minimal.x86_64-Linux is 100% reproducible | Hacker News
Incredible work from the community, especially @raboof who consistently kept pushing for the finishing line.
@tomberek shared their result of the GNOME ISO in the r13y matrix channel: Is NixOS Reproducible?
For a while now the ISO is no longer 100% reproducible, anyone have a clue what happened?
1434 out of 1501 (95.54%) paths in the minimal installation image are reproducible!
The paths are “unchecked”, not “unreproducible” - which means they might in fact be reproducible, but https://www.r13y.com was unable to verify this. This could have any number of reasons, including things like timeouts or exhausted memory or disk space.
For example, manually checking nix-build /nix/store/310735bj7rnfhy1vyrddalz2cfa9f4kl-gzip-1.10.tar.xz.drv --check suggests that derivation is actually fine AFAICS.
To dig deeper perhaps you could have a look at Builds · r13y.com