Http/1.0 would require serial connections, and the "multiple files bad" absolutely applies. Bundling and minification into a single JS file was common, even required - and I would consider that a build step. Otherwise you are dealing with all your code in a single massive file.

On http/1.1 browsers would open 6-8 concurrent connections to fetch files simultaneously. At this point, code splitting had benefits.

Most webservers now run http/2 which can fetch multiple files at the same time over the same connection. I believe it is "virtually unlimited" and the initial connection setup - which is often the largest performance hit - only happens once. At which point code splitting has such little impact on the transport layer, that it is more perfomant than transferring and loading all the code.

https://blog.vespa.ai/http2/ Has an more details as well as some load testing against http/1.0, http/1.1 and http/2