If $f(x)$ is discontinuous, then $u(t,x)= \mathbb{E}_xf(X(t))$ may not satisfy the initial condition, in the sense that the limit statement:
$$
\lim_{(t,s) \to (0^+,x)} u(t,s) = f(x) \quad \forall x \in \mathbb{R} \tag{$\star$}
$$
may not hold.  To be concrete, consider 
$$
d X(t) = d B(t) \;, \quad X(0) = x 
$$
where $B(t)$ is a standard Brownian motion on $\mathbb{R}$ and let
$$
f(x) = \begin{cases} 1 & \text{if $x\ge 0$} \\
0 & \text{otherwise}
\end{cases}
$$
 In this case, for any $t>0$:
$$
u(t,x) = \mathbb{E} f(x+B(t)) = \frac{1}{2} ( 1 + \operatorname{erf}( \frac{x}{\sqrt{2t}} ) ) \;.
$$
However,
$
\lim_{(t,s) \to (0^+,0)} u(t,s) 
$
does not exist, since 
$$
\lim_{s \to 0} u(s^2,s^{\alpha}) = \lim_{s \to 0} \frac{1}{2} ( 1 + \operatorname{erf}( \frac{s^{\alpha-1}}{\sqrt{2}} ) ) =   \begin{cases}
1/2 & \alpha>1 \\
1/2 (1 + \operatorname{erf}(1/\sqrt{2}) ) & \alpha=1 \\
1 &  \alpha < 1
\end{cases}
$$ which depends on $\alpha$.  Nevertheless, $u(t,x) \in C^{\infty}((0, \infty)\times \mathbb{R})$, and by dominated convergence, one can show that $u(t,x)$ satisfies the heat equation for any $t>0$.  Here is a graphical illustration of the solution.

![solution to Kolmogorov equation][1]

To summarize, continuity of $f(x)$ is typically applied to show that the limit statement $(\star)$ holds, which is why most theorems relating SDEs to parabolic equations assume that the initial condition $f$ is continuous.  


  [1]: https://i.sstatic.net/bPtWN.jpg