RFC 5735:
> 192.0.2.0/24 - This block is assigned as "TEST-NET-1" for use in documentation and example code. It is often used in conjunction with domain names example.com or example.net in vendor and protocol documentation. As described in RFC5737, addresses within this block do not legitimately appear on the public Internet and can be used without any coordination with IANA or an Internet registry.
> 198.51.100.0/24 - This block is assigned as "TEST-NET-2" for use in documentation and example code. It is often used in conjunction with domain names example.com or example.net in vendor and protocol documentation. As described in RFC5737, addresses within this block do not legitimately appear on the public Internet and can be used without any coordination with IANA or an Internet registry.
> 203.0.113.0/24 - This block is assigned as "TEST-NET-3" for use in documentation and example code. It is often used in conjunction with domain names example.com or example.net in vendor and protocol documentation. As described in RFC5737, addresses within this block do not legitimately appear on the public Internet and can be used without any coordination with IANA or an Internet registry.
But CloudFlare has addressed this with lots of vendors. This update is a new one.
Really the only place I saw 1.1.1.1 regularly though is to set the router ID, and making a loopback address is not the best way to do that to begin with.