A not so young anymore friend of mine has a 27" iMac Late 2013. The highest version it can run is 10.15 Catalina. To stay on HighSierra is intentional, stability is so much more important than featuritis. This user has no real need for any new features in newer macOS versions.
Certain websites (including a taxman’s site and some banks) now insist on a higher browser version. This may be solvable by trying another browser, but maybe it’s time to go up one notch.
I have seen on other Macs that going too high can be a mistake. Doing so on my own MBP made it slow or hot/loud in certain circumstances, since I upgraded to BigSur and I should have stayed with Mojave. Big Sur didn’t have anything useful that predecessors didn’t already have.
How big is the risk that upgrading the iMac to Mojave will make it less fluent to use. What’s your experience?