AHD4 has it from 1996 as a “lack of connection; a disparity: “There is a cosmic disconnect between what the voters want and what the party of the corporate interests can give them” (Bob Herbert, New York Times February 19, 1996).”

I’ll check NPA when I surface from my seasonal difficulties, but that year seems right to me as a time of origin.

1983 N.Y. Times 17 June B6/5 There is a total, absolute disconnect between the Administration and the Congress as to what the armed forces are to do.

OED has the noun form as a US origin from 1951 although I’d query their cite as a noun usage “They were not liable when local law enforcement agencies provided information upon which a disconnect order was based.” (New York Times) I’d define this as an adnoun or an adjective. And from 1958 which is more convincing “Now if someone would only invent a telephone that would sift out unwanted calls with a disconnect on the first ring. (Los Angeles Times)

Their second noun usage is the cite languagehat has above.

flynn999, your earlier cites are not the meaning the poster is asking about.

I’ll disagree the the OED and say that the 1951 citation is not a noun, it’s a verb acting as a modifier. You could rewrite it as “an order to disconnect was based.”

The 1958 cite is a noun, but in a literal, not a metaphorical sense. The OED splits these senses.

A check through Google Books finds much earlier uses in the context of telephony:

“the ‘A’ operator must be signaled when each subscribers removes his receiver, and these signals must reappear as a disconnect as soon as the receivers are replaced”

from ‘Telephony:  A Manual of Design, Construction, and and Operation of Telephone Exchanges’ by Arthur Vaughn Abbott, v. 6 p. 154, published in 1905.

There are other similar books on telephony or electrical engineering, with “disconnect” used in the sense of “disconnection” in a circuit.