I didn’t know about IPv6 leaks until today.
I am currently connecting to Proton VPN using their OpenVPN configuration file and Gnome’s built-in VPN interface. Apparently, because I don’t use the actual Proton VPN app, I am unable to prevent IPv6 leaks.
I first noticed the leak when I typed in “restaurants” in Brave Search on the Brave browser. It gave me a localized result for my actual city rather than the city of my VPN connection. I searched this in a Firefox profile using arkenfox’s user.js and it did not have any leak.
Luckily this issue can be resolved by disabling IPv6 on your browser or even your operating system, but I think mentioning IPv6 leaks (and other potential leaks in general) in an overview on VPNs is something considerable to say the least.