I own a 2011 IMAC running High Sierra. Is there anything I need to do to keep it healthy since Apple will no longer upgrade it to newer OS?
iMac 27″, macOS 10.13
Do not install and run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac? That will help keep it lean and "mean" 😉.
That being said start saving up for a replacement as MrHoffman has succinctly pointed out the issues to be expected with an 10 year old computer.
You’re going to be replacing this iMac sooner or later. A ~decade-old Mac is not going to be economically repairable, should (when) something fail.
As for your question, you can either take over software maintenance and updates yourself, or pay somebody else for that work, or migrate to a different operating system that is receiving updates when installed on ~decade old hardware.
Doing updates yourself requires substantial time and effort and skill, and paying somebody else for that assistance will be far from cheap.
Older software and particularly older networking software and older web browsers will age out, and will fail, too.
The last security update released for High Sierra was Nov 11, 2020.
Allan,
I was concerned about security so thanks for your input.
2011 IMAC running High Sierra security
