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At first glance, R22, C22, and C23 look like a compensation network around the TL431.  However, this doesn't make much sense because the TL431 is being run open loop.  Rfbt and Rfbb divide down the output voltage.  The TL431 is turned on when that reaches 2.5 V.  The TL431 is therefore used to compare the output voltage to its regulation threshold.  When the voltage is above the threshold, the TL431 turns on, which turns on the LED in the opto, which then signals the controller to stop making pulses.

A more likely reason for the R22,C22,C23 network is a little feed-forward.  This would change the signal thru the opto from a simple high/low indication to have a small transition region.  In that region, the opto will dither, somewhat proportional to the output voltage within that narrow detection region.  This was probably used for a little less ripple on the output than would have resulted from a pure high/low indication.

As for R21,D21,C21, I have no idea unless the signal at the top of D21 goes somewhere else that is not show.  Without that, all that this circuit appears to do is dump a pulse of current on the TL431 whenever it switches on.  I can't come up with a reason why you would want to do that.  In fact, that seems like a bad idea, so I don't know what the point is.