Well, being unsupported comes with the inherent eventuality that you also won’t get updates for and won’t be able to download programs you use or might want to use in the future. Nobody can say how long that will be. Depending on what you use it for, you may be ok with this. I have been using a 2010 iMac, that still works perfectly fine after a hard drive replacement. However, about 75-80% of the programs I want to use are unsupported because of how old the OS is. Even when I try to go back to earlier releases, I’m often just out of range, as their oldest release is typically OS 14 (I’m on 13.6). The fact that Apple is moving away from intel to the M series chips means the water is even more murky as to how long you will see support for intel based iMacs.
Long story short: If it does what you need it to do, there’s not much downside. Just be prepared to start seeing an increasingly longer list of unsupported updates and programs as time goes on.