Preferred way to store cards long-term?

[mrmopar]

Sounds like maybe they were early plastic sheets. Not sure of the year that they started to be more like the modern sheets, but the older ones I have seen are thicker, a little more rigid and yellow and have that melting plastic look where they stick together or stick to cards inside. Junk those ASAP if that is the case. I'm assuming the never, better sheets have had no issues noted since they were rolled out.

Someone with better knowledge of plastics could dive into this deeper, but my understanding is that newer hobby products were made with archival needs in mind (specific to long term storage of paper products within them), where as older sheets did not.