Indelible does mean 'unable to be forgotten', but you can't always use it interchangeably with unforgettable unless you change the structure. Unforgettable can be used on its own but indelible requires a verb and a noun. The trip 'left' an indelible 'impression' on me. The trip was unforgettable. It would be quite formal to use the indelible structure here, and would sound very emphatic. Speaking less formally, you could say something like: The trip gave me some unforgettable memories. The trip gave me some memories I'll never forget. Indelible might be used in situations such as: The things he saw during the war left an indelible mark on him.

They are interchangeable. However, unforgettable is more common for that context. Indelible is more commonly used for objects (markers etc) where I come from (US).

Just saying, I have probably never even seen indelible outside of a book or two. Unforgettable is wayyy more common.

Indelible is usually used in its literal sense for referring to markers i.e. pens and ink, and the marks they make. When used metaphorically it sounds quite literary and sophisticated, and more emphatic than unforgettable, even though the meaning is the same. It would probably be used in writing rather than speaking.