The proposal and its examples show static usage

a = [1, 2, 3, 4]
a[1:3]
// → [2, 4]

Eventually the question will be raised if access can be done dynamically by an expression:

Whether it's by variables

i = 1
j = 3

a[i:j]
// → [2, 4]

Or with operators

i = 2
j = 2
a[--i:++j]
// → [2, 4]

Or other expressions

i = () => 1
j = () => 3
a[i():j()]
// → [2, 4]