It is not something you will loose by upgrading the OS. The ‘Handoff’ feature is a cool gimmick for newer Macs and iOS devices it is hardware dependent from a later date. The feature is explained here: Use ‘Continuity’ to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
If you “absolutely depend” on some software that won’t run beyond OS X El Capitan 10.11, then by all means use that instead. Before upgrading, check your essential software and their websites for supported OS versions. Note that some 3rd party software will have not only minimum system requirements (like your photography software), but may also have less documented maximum system requirements (won’t run beyond a certain OS version). Which includes a Mac App Store link: Get macOS High Sierra. How to upgrade to macOS High Sierra - Apple Support This is necessary in order to boot Catalina. Installing High Sierra on these machines will update the machines system firmware, allowing it to boot from an APFS volume. Generally, the added features do not make old machines crawl, luckily, if you maintain the software in a healthy state, and keep a free space reserve on the startup hard disk drive. This usually occurs on machines that support High Sierra (macOS 10.13) natively, but whose owners have not installed High Sierra on them previously. Apple recommends to upgrade your devices to the maximum that is supported. The iMacs from that year can be upgraded to macOS High Sierra 10.13.6 at most. I have it running macOS Sierra 10.12.6 currently.
