Yes I did. It's very wordy and very lengthy and that's very nice but it's ultimately not "Proof". It's not a panel where Shanks gets outright called a swordsman, it's just your own thoughts and opinions.
This however, is proof:
This is a draft for one of the colourspreads, it's made by Oda himself. It's not a Vivre Card, or a databook, or fanart, this was by Oda's own hand.
If you have to know, it's from when Oda drew every Vice Captain with Zoro up front.
In it you have Zoro, but instead of VCs you have swordsmen, it shows the big main swordsmen character in the series, and as such, the characters Zoro has or has to surpass in order to become WSS, some of these include:
Mihawk
Kinemon
Kiku
Ushimaru
Tashigi
Vista
Cavendish
Fossa (from WB's Crew)
Koshiro and his students.
Guess who's not showing though?