The shelf life of these Thermalsil III TO-220, TO-218, TO-3 pads and the mounting kit for TO-220 - 4880SG from Boyd Laconia (former Aavid) is not documented.
As per the manufacturer:
For these insulator pads there is no specified shelf life for the materials, the mild adhesive on the AC ones (one side adhesive option) has a shelf life of 12 months,