Full Idea
Until the 1960s standard truth-table semantics were the only ones that there were.
Gist of Idea
Until the 1960s the only semantics was truth-tables
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.10.1)
Book Reference
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.14
A Reaction
The 1960s presumably marked the advent of possible worlds.