How to build an opto-isolated analog input without an input-side supply?
Linearity will be not very good in the second circuit.

In the first circuit you get quite good proportionality of photo currents at both detectors (even if they don't match very well).
This is not the case in the second circuit.

Also note that the first circuit is a linear (input) voltage to (photo) current converter. It works correctly also for input voltages that are smaller than the forward voltage of the LED.

Not so the second circuit. It doesn't work below forward voltage of the LEDs and if even if input voltage is above forward voltage, if the forward voltages of both LEDs differ only a little bit the input to output linearity will suffer a lot.