gelatin hardens as it ages. And it does so in an uncharacterized manner. As the gelatin of a matrix film ages, it undergoes changes which I don’t know have ever been studied. No reason to do so since no film maker ever thought people would store soft gelatin film for decades.
Most of those still making Dye Transfers are doing so with old matrixfilm. They are hoping that cold storage is extending their working years, but how long is it useable? The following table presents some results of testing that I performed last year using matrixfilm I have gotten from many sources. I do not know the real storage condition of the materials, but everyone claimed their matrixfilm had been stored correctly since they bought it.
What is good enough?
Can it produce a mat that can hold Kodak dyes, and transfer completely, providing a full range of tones — simple, right.
As the matrix film ages, it hardens on its own. It does this based on size of the sheet, temperature of storage, and something else — I don’t know what, but I suspect that it is the sensitizer used during the coating of the emulsion. The more senisitive to broader spectrum, the quicker the matrix suffers from “hardening of the storage” — ortho last longer than panchromatic — smaller sheets last longer than bigger. Zoing.

Once you process the matrix it can be stored, seemingly indefinitely– just don’t “poison” the mat and it will last as long as any film (my guestimate). If the gelatin hardens it can readily be softened enough to pull prints. This has been confirmed by pulling mats that were first exposed circa 1974.