Here is Szabo's first publication (occurring in 1994) regarding smart contracts - https://web.archive.org/web/20011102030833/http://szabo.best.vwh.net:80/smart.contracts.html in which "immutable" not used and "network" appears only once.

The Idea of Smart Contracts, Nick Szabo, 1997 summarizes:

Formalizing and Securing Relationships on Public Networks, Nick Szabo, 1997

The Idea of Smart Contracts is a summary a summary of a 1997 Szabo publication entitled "Formalizing and Securing Relationships on Public Networks" - https://ojphi.org/ojs/index.php/fm/article/view/548/469. As before in 1994 the word "immutable" does not appear in this publication. Szabo does speak to networked computers but the protocols revolve around computer security in terms of the encryption/decryption of messages between computers. Thus he says: "Cryptographic and other computer security mechanisms give us a kit of tools and parts from which we can build protocols, which form the basis of smart contracts." Id. under "Protocols". I deduce that the conceptualization did not envision the development of smart contracts back-ended by blockchain-type technologies or the networking of immutable data objects.