HID/FL phosphors don't seem to change if the lamp isn't used - at least not in terms of decades.
How about LED phosphors? If the lamp is stored in normal conditions (20°C/68°F with 50% relative humidity) and the electronics won't fail, will it perform like new after like three or four decades?

Why I'm asking - there may be some chemicals evaporating which might result in efficacy/colour changes etc.

Interesting question  , HID and LED filament phosphor is protected by a gas filling, but SMT diodes are not, I wonder about this also?

The thing with most of the plastics (the phosphor deposited onto a LED die is a form of plastic, after all, namely the material that holds it in place) is, they do degrade on their own. Recrystalize, oxidize, long chain molecules break, individual components reacting to each other,... Of course heat is a major accelerator, as well as any higher energy radiation (all lights may be except of red part and all shorter wavelengths). Access to oxygen or mainly the ozone or other oxidizing agents accelerates many of the degradation processes too. Plus some components, mainly softening additives do evaporate or degrade quicker, leaving the rest brittle so much it starts cracking on its own

Generally you can not stop that aging, you may just slow it down (no heat, nor UV nor intense light exposure,...).

OK, thanks.

the phosphor deposited onto a LED die is a form of plastic, after all, namely the material that holds it in place

Isn't it the most heat-stressed plastic in the lamp then? I wonder if a glass holder couldn't be used instead?
We don't have a long term experience with modern phosphors (I mean the chemistry, not the plastic). But maybe storing lamps in a freezer could probably slow down all processes hence aeging.

The phosphor in most white LEDs, are Ce:YAG, which was trailled in mercury lamps by Philips (HPL Comfort) and GTE Sylvania (Warmtone).

The phosphor in most white LEDs, are Ce:YAG, which was trailled in mercury lamps by Philips (HPL Comfort) and GTE Sylvania (Warmtone).

Interesting. That's probably why some long used LEDs tend to green out similarly to MV...?

This related to the phosphor quality and the load that the LED works. Most LEDs in lighting fixtures, are severally overdriven, in order to get a reasonable brightness, and some reaching very high temperatures (More than 100*C), in which the phosphor degrades fast.

The phosphor in most white LEDs, are Ce:YAG, which was trailled in mercury lamps by Philips (HPL Comfort) and GTE Sylvania (Warmtone).

But what degrades in LEDs is not that much the phosphor material itself, but the transparent plastic material the phosphor is supended in. The phosphor (Ce:YAG,...) is a kind of ceramic powder, which needs to be is mixed with a kind of glue aka gel aka plastic and the whole mix then "painted" on the LED itself. And the plastic goes brown after some time and exposure.
Of course, the main operation stresses, so the heat and blue light, degrades it way faster than anything else, but this degradation progresses even when stored without any power, just way slower, so it won't degrade that much in 10 years, but after 40..50 years there will be some degradation. Plus for that time some other mechanisms, not that much related to normal operation (or even those slowed down by it, like the humidity being expelled out of the materials when the thing warms up), may become the dominant ones.

Some of the phosphors are susceptible to degradation by atmospheric humidity, especially the narrowband red KSIF/PFS used in higher colour rendering LED products.  It is notable that hermetically sealed LED lamps like the gas-cooled filament types often show better long-term stability than the more common open constructions where atmospheric pollutants can cause degradation.  On the other hand, VOC's released from plastic materials of the LED packages are also highly toxic, and without suitable getters or outgassing those can also reduce the life of fully sealed LED products.