Are they still manufactured? No modern camera or flash still use that old connector. You could buy these over 40 years ago ...
Studio lights (even new ones) still have trigger inputs like this. Most studio lights have their own slave sensors, though. A complicating factor is that the modern hot shoe flashes actually send out an information-encoded pre-flash to other hot shoe flashes of the same make. As a result, you can control a bunch of slaves from the camera -- which is very cool, but it makes it harder to use "dumb" old studio flashes as they'll false trigger with the information pre-flash.
Are you saying that new flashes operating as slaves receive encoded information optically, as in, with light, and not an RF link? If so, that's interesting.
I can’t speak to studio flashes since I have no experience with them, but Nikon’s Advanced Wireless Lighting (AWL) system, introduced in 2003, uses the xenon flash tubes themselves to send data to slave flashes (the slave flashes only look for the data in the infrared spectrum). For clarity, I shall henceforth use the brand name “Speedlight” to refer to the devices, and “flash” to mean an actual burst of light from the xenon tube.
As best I can tell, the system works like this:
1. The photographer presses the shutter release.
2. The master Speedlight (which is either one built into the body, or one connected via the hot shoe), based on the settings you set in the camera menus, sends an initial optical data burst instructing all the slave Speedlights to pop a metering flash.
3. The camera’s metering sensors observe the scene without and with the metering flash, to determine the amount of light the Speedlights are adding to the scene, and the camera calculates the correct exposure and flash settings.
4. The shutter opens.
5. The camera commands the master Speedlight to send another optical data burst telling the slave Speedlights their specific settings.
6. All the Speedlights pop the exposure flash in sync.
7. The camera waits until the selected shutter speed has elapsed.
8. The shutter closes and the exposure is complete.
When using rear-curtain flash sync, the order is slightly different, in that the step 7 moves ahead of step 5, such that the metering flashes still occur right
before the shutter is opened, but the actual exposure flash pops at the
end of the exposure.
Because the second data burst (step 5) happens with the shutter open, then when a) shooting at very, very close range, AND b) using a built-in Speedlight as the master, AND c) manually setting said built-in Speedlight to “off” (i.e. “please don’t contribute light to the scene!”), the data burst will cause slight unwanted light in the image. Nikon actually sold (sells?) a little IR filter to dangle in front of the built-in Speedlight, to permit the data burst in IR to pass, but blocking the unwanted visible light.
And all this happens literally in the instant between pressing the shutter release and the shutter opening, which is something like 1/10 second perhaps (it feels instant in practice), assuming focus lock was already achieved and the Speedlights’ capacitors are charged.
The system allows for 4 channels (so different photographers don’t step on each other’s toes), and within each channel, 3 groups of Speedlights. So assuming a camera body with a built in master-capable Speedlight, it means you can individually set the flash exposure (manual, auto, auto+compensation value, and trick features like repeating strobe, in some models) for the built in Speedlight and for each group of slave Speedlights. Each group, in turn, can have as many Speedlights as you want.
I own 4 such Speedlights and often use one on-camera and let it control the other 3. So I can do backlight, side light, etc with quite fine control of each Speedlight to compose an image.
Now, in the past year or two, Nikon released an RF version of AWL, which they are imaginatively calling Radio-Controlled Advanced Wireless Lighting (“radio AWL”). I haven’t had a chance to use it. I’m not entirely sure what problems it solves, since the xenon/IR optical system works so very well (in my experience, you have to really work to make a slave not see the commands, it works fine around corners and stuff - the IR receivers must be very, very sensitive!). The only thing I can imagine it adding is feedback on the slave flashes’ charge status, which would let the camera know to delay shutter release until they’ve recycled. (Or let it go Canon-style and allow an exposure with incompletely recycled flashes, compensating the reduced lighting through ISO change or something.) But because radio AWL remains compatible with optical AWL (in that both can be used simultaneously, since all radio-AWL-capable hardware is also optical-AWL-capable and allow mixed use), I suspect that little, if anything, has changed beyond the actual communication medium.
Of course, now this has me curious as to what the data packets look like, etc. Maybe someday I’ll lash up a phototransistor to my scope and see what I can find, assuming nobody else has done this already...
P.S. It just dawned on me that Nikon uses the word “commander” for what I’ve called the “master” above. I think I can’t be bothered to edit it all. I don’t think this affects clarity, other than knowing what it’s called in the camera menus.