Auto Tune Plugins
When it comes to Auto-Tune plugins on Linux I've only found two, GVST GSnap and Graillon. Both are free, however Graillon offers a FULL Edition which comes with three further enhancements.
...
GVST GSnap
GSnap is nice and simple, and comes with a few presents such as "Subtle".
Check it out at https://www.gvst.co.uk/Downloads/GSnap
Graillon v3.0
Graillon looks more cosmetically pleasing, and obviously the full edition has more features than GSnap. At the time of writing this it was for sale at $29 (£22.30).
Check it out at https://www.auburnsounds.com/products/Graillon.html