an ill-kept promise

It was not your invention, or rather you may have arrived at it independently but the phrase "ill-kept promise" to convey that meaning is widely used.

Oh! It's surprising!

A promise we make can be rash/broken/unfulfilled/empty/false/hollow, but does an ill-kept have exactly the meaning of a promise that I still have some chance to fulfil/honour/keep?

Yes, it does. The use of ïll-" as a combining form is quite common, and has been in use literally for centuries. There is a character in the Arthurian romances--Sir Breunor--who wears the jupon, the "jersey" his father was wearing when he was treacherously murdered. He has sworn to wear it until he avenges his father's death. Sir Kay, always a snide and sarcastic bastard, calls him La Cote Mal Taile. This is usually translated within the text as "The Ill-fitting Coat." This appears in Malory's Le Morte d'Arthur published by William Caxton in 1485. It was very likely in use (the "ill-" combining form) far earlier than that.

A common usage, for example, will be to describe someone's rash act as ill-considered. A bed not properly made up will be called ill-made. It is a very versatile usage.

Yes, I suppose so.

A long-neglected promise? An almost-forgotten promise?

Thanks all!