If you're looking to run a commercial application that requires one of the three most recent versions of macOS, you're out of luck, unless you buy a new(er) computer. (Microsoft 365 / Office and the Adobe applications are examples of commercial applications that fall into this category.)
If you upgrade to High Sierra, there are some applications that could let you extend the useful life of your Mac.
- The Firefox Web browser still gets updates for High Sierra.
- LibreOffice 7.4.7 will work with many Microsoft Office files. It's currently very visible on the Downloads page, although it soon may rotate off into the archive.
- GIMP is a Free and Open Source photo editor with many features.
- Carbon Copy Cloner 5 and SuperDuper! will run on High Sierra. Whether you use Time Machine, or use one of these clone backup tools, you should be backing up your files on a regular basis.
