Valid question. I knew that someone had to have done it - a quick search shows ifconfig.me as being such a service, but wanted a simpler one that anyone can hack on.

echoip.xyz is written in pure PHP and can be self-hosted without any dependencies other than PHP, and is very transparent in what it does.

ipconfig.me also outputs "Moved Permanently. Redirecting to https://ifconfig.me/%" as of right now with the "curl ipconfig.me" command.