How do I find which app/software contains the legacy system?

A new information popped up on my Mac this morning: "Your computer contains legacy system which will not be supported in the future updates." (or something to that meaning). Sadly, that note is note accompanied by any information as to which app/software (among the many on my Mac) it is. How do I find the problematic piece of software to a) get rid of it or b) replace it with a newer version?