Simple ten-year-old 64-bit Cocoa apps will generally run on modern macOS with no changes. The more complex your app is the more likely it is to have been broken

Simple ten-year-old 64-bit Cocoa apps will generally run on modern macOS with no changes. The more complex your app is the more likely it is to have been broken by an OS change since macOS is not especially concerned with backwards compatibility, but the actual APIs you're programming against won't have changed much in that time.