That's not true at all. Any number of seemingly-benign websites can be compromised and loaded with webkit exploits that will instantly pwn any non-updated machine. The fact that you're visiting MacRumors is evidence that you visit sites beyond the most secure services from Apple, Google, etc.
People have a misconception that malware is only related to shady and illegal sites, but it can be anywhere--- lets say you want to convert 5 liters to gallons and you click on the first website that shows up in the search results, promising to be an easy measurement conversion calculator. If it's your unlucky day and you're running an old web browser or old OS, congrats on becoming a new spambot or cryptominer.
Outside of web browsing (which constantly brings new remote code to your machine), it's probably safe to use some non-web internet services on an old/nonsupported machine if you're behind NAT.