Do you mean to say I shouldn't use the ground fill onto or near the optos. sorry, I couldn't get it.

It's mainly the ground fill directly under the optos, which is unwanted. Nevertheless, there should be a ground plane whereever digital signals are routed! You mean, this can be hardly accomplished? Well, now you know what our problems are...

What is an opto used for?
To prevent noise from propagating from one device to the next. An opto makes this by transmitting the signal via a modulated light signal. By this a total galvanic isolation can be achieved, means noisy signal at one side of opto coupler cannot run to the other side. This is what the word 'barrier' tries to say.

But, while an opto provides an optimal barrier against 50Hz noise, interference of higher frequencies can jump over the barrier. Why that?
Because for high frequencies input to output stray capacitance comes into play. This stray capacitance provides a rather low impedance for high frequencies, means if a very fast glitch hits the input side of opto, then a certain portion of this glitch will appear on the output side. For very high frequencies the opto coupler does not provide any longer a good barrier against interference.
Only by minimizing input to output capacitance of an opto coupler, the barrier will work as a barrier. But not only input to output capacitance of opto coupler itself is relevant, but also the stray capacitance between any signal applied to input pins and to output pins! And because of this, it's extremely important not having anything routed between input and output pins of an opto coupler!!
There should exist a barrier also on the PCB, namely between all input related signals and output related ones.

What to make with the ground plane arround an opto coupler?
A ground plane is needed whereever digital signals of your microcontroller circuitry are routed. So, the ground plane of your microcontroller board should be extended up to those pins of optocoupler which are connected to your microcontroller circuitry. But don't route this ground plane under the opto couplers, between their input and output pins. And do never route this ground plane under signals, which are connected to the other side of opto couplers, the side which has no connection to microcontroller circuitry! Think of the needed barrier...

Kai