Any thoughts on using this (or software like it) to beef up the security of old MacOS versions? I have an old MacBook Pro that can’t run the latest macOS anymo

Any thoughts on using this (or software like it) to beef up the security of old MacOS versions? I have an old MacBook Pro that can’t run the latest macOS anymore. I would rather not toss the machine but I’m skittish to use it without security updates. But I might be willing to use it if I could secure it “enough” by monitoring what it does on the Internet, and by not putting sensitive data on it - for example I wouldn’t log it in to my iCloud and would only use it for some non-sensitive tasks (I use TaskWarrior as a task tracker. This works fine on an old Thinkpad but that thing has a miserable screen.)