Sequence diagrams are very useful, but UML just adopted a representation that already existed decades before. For example, the original 1981 TCP specification in RFC 793 [0] has ASCII sequence diagrams (see figures 7 to 14, etc). I don't know if they predate that - less from that era is online - but I wouldn't be surprised.

[0] https://www.rfc-editor.org/rfc/rfc793

Edit: there are a form of sequence diagram in an early FTP specification from 1971: https://www.rfc-editor.org/rfc/rfc172.html