In terms of handling your practical problem, you just have to enter "http://" manually in the address bar in Safari in order to use http instead of https. However, the server has a major say here, and they could actively be redirecting users to https using HTTP redirects. This is very common and often a good security practice. This means that Safari (and other browsers) will automatically switch to https.
In addition some sites might indicate to browsers that they only permit https queries in the future using the HSTS header - this is a practice that is becoming more and more common, and is also most often a good security practice.
You could use a different browser that has updated TLS functionality and CA repository so that you get https access working again for your sites. However, that would still leave your computer exposed due to the lack of operating system updates.
A third option is to use a browser within a browser. I.e. it is possible to access a site via http that will essentially run a browser on a remote server whose user interface is displayed and operated within your browser. Technically the connection between that second browser and the site in question would be https, but you would access the result of that interaction though a http connection. Common examples of "browser within browser" are Browserling and BrowserStack - although you would have to test yourself if those can actually by accessed with your specific setup as they typically assume usage by https.
The best way to proceed here is to upgrade your system - operating system and browser:
I know that you demand that the we do not answer this, but for the sake of other's that might read this, I must answer that the correct way to proceed is to upgrade.
The reason for this is that Yosemite no longer receives security updates, and is thus a target for explotation. There's a realistic risk that your computer will get infected, hacked, taken over, etc. Even though you are "just browsing" and not logging in, that risk is there.
As I understand from your comments, you're running on old Apple hardware for which no recent macOS updates exist. I would suggest upgrading to an alternative operating system, such as for example Linux.
Even though you do not handle financial information, you might have other information on your computer, that you would like to keep private. Even if that is not the case, then you do expose other user's to risk when you do not update your own computer. Primarily because your computer could be taken over and used for nefarious purposes against on other user's on your local network, or over the Internet - for example by participating in DDoS attacks. You also run a financial risk in that malware some times install Bitcoin (or similar) miners, which will mean that your computer will start running a larger electricity bill than usual.
