It is mid-August 2021 and I am still happily using OS X 10.9.5 Mavericks as my main OS on my desktop computer (Mac mini late 2012) and laptop computer (MacBook Air mid 2013); actually the hard drive of the latter is just a clone of the former, cloned with SuperDuper!
I would say that people probably shouldn't still be using Mavericks like me unless, like me, they are a Mac power user and are happy with Mavericks. I use MacPorts to install and use the most recent versions of the bash shell and command-line tools, recent versions of Python, etc. I also use VMware Fusion Pro to run other operating systems, including macOS 10.12 Sierra, within Mavericks as host OS. I use the application firewall Little Snitch for added security. I keep JavaScript disabled in Safari and never use Safari to browse the web; all web browsing is in Firefox ESR (extended support release). Also for security, I don't connect to public wi-fi hotspots from Mavericks.
I told myself that I would look into upgrading the OS when Firefox ESR stopped receiving updates for Mavericks, which was due to happen in July 2021, but a week ago another update was released. I suspect my Mavericks era may be over by the end of the year, but not because I'm not happy with my setup. I'll just keep riding this wave as long as I can.