Note that stock in U.S. and EU is up to date now.
U2F CHECK_ONLY command
U2F Zero did not correctly handle checking if a key handle was generated by the particular U2F Zero token. This was
initially only an issue when a user used U2F Zero with multiple U2F tokens on an account. The CHECK_ONLY U2F command would be used to see which key handle to use with the U2F token that was plugged in.
Later, applications like Firefox and SSH PAM plug in, started always using the CHECK_ONLY command, causing more of an issue.
This issue was fixed back in November 2017 (see aafd053 and c04163b). Due to U2F Zero's low volume production and international shipping costs, it has taken a while to update EU stock. When CHECK_ONLY later became more of an issue, all EU stock was halted. All stock should be up-to-date by mid June 2018. U.S. stock is up to date.