MATLAB is unable to prove that y is always greater than or equal to 0 because y is not always greater than or equal to 0. Here is a case where it is:
x = 1;
y = x^2
y >= 0 % true
Here is a case where it is not.
x = 1i;
y = x^2
y >= 0 % false
If you told MATLAB that x was real, it would help.
syms x real
y = x^2
isAlways(y >= 0)
