Optocoupler ac model ltspice
The optocoupler 1st-order model is a simple current-controlled current source whose output is decoupled by a parasitic capacitor. This is what has been represented in the post. This capacitance needs to be characterized and accounted for when adopting the compensation strategy. The below circuit shows how to extract this parasitic element. More details can be found in the APEC 2018 seminar that I taught in San-Antonio (TX):

When realizing a type 2 compensator (1 pole at the origin, 1 zero and 1 pole) with a TL431, you form the second pole by decoupling the feedback pin of the controller to ground via a capacitor. This is \$C_2\$ in the below picture:

However, it is important to realize that the optocoupler pulling the FB pin to ground adds its parasitic capacitance to the capacitor already in place. It does not add a second pole (at least in low frequency) but simply shifts the original pole: \$\omega_p=\frac{1}{R_{pullup}(C_2||C_{opto})}\$. As such, once the type 2 pole position has been determined and a capacitance calculated, the final value connected to the controller must account for the optocoupler capacitance. For instance, assume you calculated a 4-nF total capacitance and the opto already contributes 2 nF, then the final capacitor will be 2 nF. As indicated in the comment, I was originally considering the optocoupler pole as an added one but it is not the case as explained in the above lines. The extra block X4 can thus be omitted.