Years ago I swore by KEM, such as these bridge Arrows I've probably had for 10 years. Back then somebody got me about 6 of these thick plastic card-sized pieces, so I put these on top then put rubber bands on the box. This thread inspired me to look at them. The gold deck here is flat and mint. The grey deck? The familiar slight KEM bow.
I have WSOP cards from the 2008-12 era, when
USPC acquired the Bicycle brand. Most also have a slight bow in the boxes with those plastic pieces or random chips on top. Also have a World Poker Tour KEM deck that's about 10 years old, it's bow is side-to-side (like a subtle half-pipe), like people were bending them pretty serious while looking at them lengthwise, rather than top to bottom.
Fast forward to 2016-24, and my Copag WSOP cards are flat and mint, just kept in the cardboard boxes the single decks came in.
Full disclosure, I live in Florida where the humidity is 70%+ for 4-5 months out of the year.
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