I am using manjaro, kernel 5.4.95-1, kde plasma version 5.20.5, framework version: 5.78.0, qt version: 5.15.2 

The problem is :

the proxy applet in the settings does not honor pac file for auto configuration.

The pac file as follows :

function FindProxyForURL(url, host) 
{
  if (
  
    dnsDomainIs(host, "ieeexplore.ieee.org")     ||
    dnsDomainIs(host, "www.scopus.com")          || 
    dnsDomainIs(host, "onlinelibrary.wiley.com") || 
    dnsDomainIs(host, "link.springer.com")       ||
    dnsDomainIs(host, "sciencedirect.com")       ||
    dnsDomainIs(host, "dl.acm.org") 

  ) 
  {
    return "HTTP username:password@ip:port"
  }
  else 
  {
    return "DIRECT";
  } 
}

This file is tested with pacparser package of manjaro and seems working. It is deployed in firefox and firefox behaves as expected and directs specified urls  through proxy. 

In KDE network settings proxy user interface "Use proxy auto configuration URL" is filled as "file:///etc/proxy.pac". The file is readable by all. 

Firefox proxy setting is selected as the same as system. The computer is rebooted . 

The firefox or other browsers in the system such as chromium or vivaldi etc. can not direct the specified addresses through proxy as it has done before. 

SUMMARY:

KDE setting network proxy interface does not honor pac file settings.