MaskedNybbles

dig -6 +short @resolver2.opendns.com myip.opendns.com AAAA

Note: You have to ensure you are actually contacting the server with IPv6.

I worry less about the service breaking, changing, or otherwise disappearing, over a random website.

EDIT: Also what was said in a sibling comment.

I saw it used in another comment, and am already aware of the use of curl for such a task, but choose to query DNS services instead—especially in scripts.

#!/usr/bin/sh  
dig -4 +short @resolver2.opendns.com myip.opendns.com  

#!/usr/bin/pwsh  
Resolve-DnsName -Server resolver2.opendns.com -Name myip.opendns.com -Type A | % { echo $_.IPAddress }  

There should be an IPv6 resolver, but I don’t remember and am currently unable to test. My PowerShell skills are also effectively non-existent.