The Hive account@bitcube account has been idle since 2016, and that happened suddenly without notice (prior to going offline, there was active engagement). There was a theory circulated that the person behind it may have died without giving anyone the keys, but I have no idea if that is true.

The Hive account@mottler account was inactive since the start of the Steem chain (never engaged to my knowledge) before recently powering down.