I have and use the same vintage 2011 iMac and our best protection is using our brains as to what you click on and go to and not clicking on dubious sites if you don't know where they are going to take you. Better than any software!!!
Your Mac OS will probably still provide you with warnings if you happen to attempt to go to a nasty site.
Yep. My 2014 Mac Mini has never been infected with malware. The same for my previous Macs. Just safe practices is all that's needed, no software required for security, other than the security built in by Apple.