You could say: the car was rusty with disuse. Or the farm had fallen into disrepair with the old man dying. Or: it was not used, so it lay around like a forgotten memory. But it would be a very poetic thing to say it had fallen into disuse...not normally used at all, but it has a lovely ring to it.

Obsolete or antiquated.

Fallen into disrepair. if you want to use fallen. or is obsolete, outdated, deprecated, discontinued,. plus more or for idioms/colloquialisms : past its best, past its sell by date, knackered, kaput, plus more

Outdated, or obsolete? But I recommend you look them up and just confirm whether they have the meaning you are looking for.