This backpack was purchased in 2015 and has been stored in a closet for the past few years. I've noticed that the rubber layer on some zippers has turned yellow and is peeling, and there is also some coating peeling on the inside. What could be the reason for this, and how should I address it? Thank you, everyone.
What happened to your backpack is just ageing.
In 8 years of use and/or storage rubber will go yellow and start peeling. This is to be expected with these materials and not much can be done.
The time span depends on the quality of the pack, the time it was in storage before you bought it, how you stored it, the temperatures, humidity and likely some more details.
It is very unlikely that you can get it back to its original condition, you might be able to get some use out of it if needed.
If the waterproof layer on the inside is damaged you might be able to replace it by clearing all off and applying a new layer, which is a lot of work and might not work. The easier version is to line it with a bin liner.
The damage on the outside is harder to work around, either it is beyond what you are willing to walk around with and you ditch the bag or it is still within acceptable limits and you use it as is.
If all of the bag was in perfect condition and only one part failing you would be right to spend time or money on it, but with the pack failing in several spots, in several ways, I would not spend money on it.