Old Magic is a system of Functional Magic that has been mostly or entirely replaced by a newer system of magic, which might be refined from it or completely unrelated. There are various reasons the old system may have fallen out of favor:

Its secrets were kept by only a few, and are now mostly lost. Its use depends on having a connection with unpleasant or unsavory gods, especially The Old Gods who nobody wants to have relations with anymore. It has an extra cost compared to new magic, such as