Thinking you mean 8.F.B.4 for the first question, the initial value of a function is its output when the input is 0 so yes, that is usually going to be the y-intercept.
And, thinking you mean F.IF.A.3 for the second question, I think the initial value is still going to be f(0) if f is defined at 0, but notice the term “initial value” is not used in this standard, and the domain might not contain zero. For example, you could have a sequence a_1, a_2, . . . whose domain is the set {1, 2, 3, . . . }. I think in this case you would call a_1 the initial term of the sequence to avoid confusion.