Since Costco is a "private club" that is not open to the public, they fall outside of the laws of most states with regards to posting requirements. I very much doubt that Costco has changed it's policy. My bet is that the store opted to revert to a "don't tell unless asked" approach, most likely due to negative feed-back regarding the sign/policy.
Big Dawg No. 1431
Carolina Glocker No. 1431
"Freedom is a system based on courage" (Charles Peguy)
"Know where the attack against you is likely to come, whether on the street or in court, and have a proven counter already in place" (Mas Ayoob)