onsemi lists the 4N35SR2M as Active, so there is no end-of-life notice to track for current designs. ROHS3 compliant — no exemption expiry to watch.
4170Vrms isolation — what it buys you
The 4170Vrms isolation rating is the reinforced-safety barrier between the input LED and the output phototransistor. For a 230VAC mains-referenced circuit, this gives you a 1.5× margin above the peak 325V line voltage — enough for basic functional isolation in an industrial PLC input module or a solid-state relay driver.
If your load needs 5mA, you can drive the LED at 5mA and still have margin. The 2µs typical turn-on and turn-off times are fine for 50/60Hz line sync, relay coil switching, or reading a pushbutton — but they will not handle a 100kHz PWM signal.
Temperature range and package — board-fit reality
The 6-SMD Gull Wing package is hand-solderable with a fine-tip iron — no hot-air station required. The base pin is brought out, which lets you add a base-emitter resistor to tailor switching threshold or speed, a common trick in noise-prone environments.