[Update Nov 24] Red Familiar Cards have been re-enabled in the Cash Shop (for non-Reboot worlds only, as intended) as the issue has been resolved. Those that used Red Familiar Cards while this issue was present will be compensated with the same number of Red Familiar Cards as they used in the Gift Drop menu. This compensation must be claimed prior to 12:00 AM UTC on December 8. Thank you for your patience.
In the wake of the v.218 Update, an error was discovered with Red Familiar Cards, where Familiar Potential was not properly being reset. In order to minimze the impact of this issue and make fully resolving it easier, Red Familiar Cards have been removed from the Cash Shop until further notice. We will work to resolve this issue as soon as we are able, and will let you know when they become available again.
-The MapleStory Team