What options exist for keeping it safe and functional?
Safe
Unsupported OSes, by their very nature, are not patched with fixes for known security vulnerabilities.
So there will always be some degree of risk in running an older OS, depending on what vulnerabilities there may be; how readily they are exploited; the extent to which old Macs might be a target; and the way that you use your Mac.
Using OCLP, to upgrade to a newer OS that does not normally support your hardware, is an option: though there are concerns about security, given that the OS is already ‘compromised’ to make it work; and new firmware updates that accompany new OSes are not applied.
Putting a number to these risks is subjective, and ultimately something that you will need to weigh up.
Functional
Your Mac will continue to run Catalina until the hardware fails. Apple isn’t preventing you from using it. At this time, there’s plenty of software available to download that still runs on Catalina; though that list will reduce as developers start to introduce code that relies on newer OS features. So start downloading now!
I’d certainly look to see if there’s a decent (secure) browser that runs on Catalina with a more recent version than Catalina’s Safari c. 2020.
However, your assessment of how functional the Mac is will be coloured by any obstacles to your use: — the inability to use current software versions; limited battery life, peripheral connectivity, and processing power. (It’s a decade-old 2-core Intel CPU. Geekbench puts it at 1009/1994, compared to an M4’s score of 3699/13592…)
You could install Linux on it; though this might require a bit of tinkering and getting used to a new environment/software.
