Safari 15 on macOS not connecting to unsecured http sites

Solution 1:

UPDATE

Given your update and the Wireshark dump I requested, it is clear that the problem is not with your Safari browser. The browser does send the request for the site in the HTTP protocol (non-encrypted). However, it never receives a reply from the server.

You need to look at any software you have installed that could interfere with network requests, devices on your local network (routers, firewall, VPN gateways, etc) as well as what your internet provider does to your traffic. If you have a secondary computer, tablet, etc it would be worth testing that on the same network.

OLD ANSWER

I'm not sure I'm convinced that this is caused by the automatic HTTPS upgrade (I cannot see why it would) - but if you believe this to be so, I would suggest trying to disable that feature.

First you need to enter Safari Preferences, click on the Advanced tab and then put a checkmark for Show Develop menu in menu bar.

Then from the Develop menu in Safari, under the Experimental Features submenu, you can disable Automatic HTTPS Upgrade.

If you haven't got this option, I would try installing Safari Technology Preview and try the same!