When I was a Canon shooter I made use of the Safety Shift custom function (C.Fn 1-6 on the 7D). The feature works in both shutter and aperture priority mode. When shooting shutter priority if the correct exposure cannot be achieved it would start to reduce the shutter speed until a correct exposure can be achieved.
For example, say I was shooting a fast moving jet and have shutter priority set to 1/1000s and ISO 200 on an f/2.8 lens. Against the sky I’m getting an aperture of f/5.6 and I’m happily shooting away. The aircraft dives down and the sky in the background is replaced by ground so I’m getting a lot less available light. If this happens very quickly and I’m not expecting it I might not be ready to change ISO. The camera can’t expose the image even after opening the lens to f/2.8 so safety shift steps in. It will reduce my shutter speed down until it can get a workable exposure, for this example I’m going to say 1/500s.
I’ve got my shot and it’s not massively underexposed but by dropping the shutter speed to 1/500s I may not have the photo I wanted.
Nikon doesn’t have safety shift, my research before making my move had highlighted this, but they do have auto ISO. I’m generally not a fan of auto ISO but the way Nikon implements it is very interesting. They let you set a maximum ISO setting and a minimum shutter speed giving you a good degree of control about what setting will be used.
Going back to my example but this time shooting Nikon and with my auto ISO set to a minimum of 1/1000s and a maximum ISO of 400. The camera will use ISO 200 I’ve got set when shooting against the sky but when the aircraft dives down rather than reducing the shutter speed it will keep that constant as I’ve set my minimum value to the same as my shutter priority, instead it will raise the ISO to 400. This gives me my shot without having to reduce my shutter speed and by putting a realistic maximum ISO setting I can stop auto ISO from selecting a high ISO setting and introducing more noise into the picture than I’m willing to accept.
It isn’t an alternative to understanding how to use ISO settings and I would recommend against using it with a large difference between you settings and the maximum ISO / minimum shutter setting but if correctly used it can be a handy setting for when you are shooting in conditions of rapidly changing light with little to no time to adjust your settings.