1.1.1.1: Fast, privacy-first consumer DNS service

It never was perfectly valid. That blog post is incorrect, and network engineers are perfectly fine arguing against that practice. The IP address 1.1.1.1 was reserved by APNIC and now belongs to the APNIC and Cloudflare research project.

Assigning an IP address you don't own on a local network usually means that you cut off access to the actual owner of that address. You might not (immediately) notice it because you don't need to access anything that's located there. But it will set you up for unpleasant surprises in the future when your users (or yourself) want to access a resource that happens to be located there.

RFC 1918 <https://tools.ietf.org/html/rfc1918> provides explicit IP ranges you should use for private resources (10.x.x.x, ~172.16.x.x, 192.168.x.x), which are not routed over the Internet and where your organization is responsible to avoid IP address conflicts.