Isn't that just someone putting some weird thing in some oddball BASIC implementation? I don't think any of the popular ones did anything like that.
A$ = "ABCD"
MID(A$,2,2)="12"
PRINT $A
>A123
x = [1,2,5,6]
x[2:2] = [3,4]
print(x)
(Slice notation can be used on any collection, not just lists, but you can't use it in assignment like that for strings because they're immutable, so you'd have to convert to a list of characters and back.)