You pose an interesting question!

I had to do some homework to understand your drift.

"At very low illuminance levels, film responds less than usual. Light can be considered to be a stream of discrete photons, and a light-sensitive emulsion is composed of discrete light-sensitive grains, usually silver halide crystals. Each grain must absorb a certain number of photons in order for the light-driven reaction to occur and the latent image to form. In particular, if the surface of the silver halide crystal has a cluster of approximately four or more reduced silver atoms, resulting from absorption of a sufficient number of photons (usually a few dozen photons are required), it is rendered developable. At low light levels, i.e. few photons per unit time, photons impinge upon each grain relatively infrequently; if the four photons required arrive over a long enough interval, the partial change due to the first one or two are not stable enough to survive before enough photons arrive to make a permanent latent image center.

This breakdown in the usual tradeoff between aperture and shutter speed is known as reciprocity failure"

Good news for digital camera users

Reciprocity failure is not a problem with digital cameras, however noise can be.

Bad news for digital camera users

Long exposures on digital cameras can produce visible noise, which looks a bit like the grain you see in a fast film image. This noise or grain is usually most noticeable in plain areas of the picture.

Very interesting topic and answers!

I would have been glad to read them 40 years ago when I unwillingly discovered reciprocity failure and cross curves by the practice;)