Steve Souders' 2007 book "High Performance Web Sites" made domain sharding gospel. Split your assets across multiple subdomains to bypass HTTP/1.1's brutal 2-6 connection limit per hostname. Every performance-conscious team adopted it.
HTTP/2 arrived in 2015 with unlimited multiplexing over a single connection. Domain sharding became instantly obsolete.
Nearly a decade later, research shows 36-72% of sites still trigger unnecessary connections through legacy configs. Under HTTP/2, those extra domains don't unlock parallel downloads. They break stream prioritization, force redundant DNS lookups, and multiply connection overhead. Your old optimization is now your bottleneck.
Infrastructure doesn't automatically shed yesterday's fixes. You have to actively unlearn them.