Vlad While I also agree with this in concept, the reality is your iMac G3 will be unable to browse any modern website thanks to ECMAScript 2015, TLS, new root CAs, and not enough memory. Even my 2011 iPhone 4 cannot load any pages any more without the use of a proxy. That proxy is https://github.com/atauenis/webone and it solves all the things I just mentioned. I am speaking from experience when I lost my phone and had to get by with the iPhone 4 for a couple of days.
Even if Kagi worked over insecure HTTP with no CSS or JS, and even if my iPhone 4 could set Kagi as the default search engine (it can't), none of the search results Kagi presents would load, making Kagi not very helpful on its own. Using a proxy is mandatory anyway.
So it's best to use reasonable security defaults. People who like old computers can let the proxy do the work. 👌