Over 80% of the servers support IPv6 but only the Linux & Android apps and the browser extension support it at the moment.
The Linux app supports IPv6 over the VPN but still connects to the VPN server using IPv4.
It's possible to make changes to the WireGuard configs that you can get from https://account.protonvpn.com/downloads to enable IPv6.
An easy way to test if a server supports IPv6 is to use the browser extension and then go to myip.wtf or test-ipv6.com, or you could check the list at https://mikaeldui.github.io/proton-vpn-server-list/.