Hmmm. That is a new one I have not previously heard of. I knew of data degradation due to random UV in ambient or artificial lighting causing problems, but since most of the EPROMs and EEPROMs had at least an adhesive label pasted over them I thought that the particular problem you mentioned was alleviated in that regard. Plus, all the boards I have stored away are in nonstatic sleeves, in foam-padded containers that have not seen any light whatsoever for many years, so is that still a concern? Bit rot from simple age?
“Bit rot from simple age? “
The EPROM is a charge storage device. Each bit is a CMOS capacitor that is charged by the programming voltage. When you read an EPROM it check to see if it’s above the “1/0” threshold voltage. If it is, it is read as a 1. If not, it’s a zero. When you erase an EPROM with UV, the light causes a temporary break down of the insulating layer which sets all cells back to zero. With no light you still have a very small leakage of the cells because no insulator is truly perfect. Most EPROMs are designed around a 10 to 30 year minimum retention lifespan. Once you get past the rated lifespan of that chip then the “1” cells start getting to a low enough voltage that they start falling below the “1/0” threshold. At that point the computer will become unstable, if not totally unusable. In real life the leakdown rate varies form IC batch to IC batch for the same class of chip. For a chip with a 15 year rated life some may have 50+ year lifespan, others barely get the 15 year rated life. The rated life is the minimum that quality control requires it to be able to achieve.