Unironically diode-to-photoresistor optocouplers are used in some compressors and filters to get that soft analoguey sound (as they are plain slow to react to signal). Which is kinda a supply problem as material they were made with was banned for toxicity

True. LDRs (photoresistors) are however still very easy to find on the usual online suppliers (Ebay, Aliexpress etc.) although they're banned in some areas for containing cadmium. I've used them successfully in LED+LDR arrangements to emulate Vactrols in vintage-style audio compressor circuits. To replicate the vintage compressor slow attack, in theory one should use a filament lamp+LDR pair so that the lamp slower attack and very slow decay, compared to a LED, would give that particular curve. However this is something that can be easily replicated using a RC network within the circuit that drives the LED.

>diode-to-photoresistor optocouplers

The crude (but wonderful sounding) type like they have in vintage Fender tube amps are used to effect the signal, not pass the entire signal.

Can't read the article but I assume these Audio Optocouplers are intended to pass the entire signal in an acceptable way.

I don't think they would be very good for T & M use either unless you were desperate for isolation.

I found great use for the regular 4-legged kind of precision non-audio optos, to isolate the physical relay logic from the audio power supply for much lower noise floor under high-gain channel-switching applications.

> The crude (but wonderful sounding) type like they have in vintage Fender tube amps are used to effect the signal, not pass the entire signal.

Yes, I was referring to the use as variable resistors in compressor circuits; the sound wasn't affected at all but the slow attack-release induced to the dynamics contributed to that particular character in vintage guitar amplifiers.