Generic 433MHz receiver, how to derive output signal voltage
the output circuit makes use of a duel opamp (LM 358) but my
understanding is too flakey to derive the output voltage from this
arrangement.

LM358 datasheet page 10:-

This tells you that maximum output voltage is Vcc - ~1.2 V = ~3.8V with 5 V supply. The ESP8266's rated maximum I/O input voltage is 3.6 V, however it probably has protection diodes that don't start conducting until pin voltage goes ~0.6 V above the power supply voltage, ie. 3.3 + 0.6 = 3.9 V.
So people who are using this combination without level shifting are just getting away with it if the power supply voltages are accurate. To be safe you should lower the output voltage by at least 0.5 V using eg. a voltage divider made from two resistors.