This is from the first edition free PDF online of The AWK Programming Language:
"Associative arrays were inspired by SNOBOL4 tables, although they are not
as general. Awk was born on a slow machine with a small memory, and the
properties of arrays were a result of that environment. Restricting subscripts to
be strings is one manifestation, as is the restriction to a single dimension (even
with syntactic sugar). A more general implementation would
allow multi-dimensional arrays, or at least allow arrays to be array elements."
"Associative arrays were inspired by SNOBOL4 tables, although they are not as general. Awk was born on a slow machine with a small memory, and the properties of arrays were a result of that environment. Restricting subscripts to be strings is one manifestation, as is the restriction to a single dimension (even with syntactic sugar). A more general implementation would allow multi-dimensional arrays, or at least allow arrays to be array elements."
Stable release - SNOBOL4 / 1967
https://en.wikipedia.org/wiki/SNOBOL