The extensions used to store the running version of steemd that signed the block, as well as the upcoming hardforks that a witness has accepted (if any). But since HF23 this information is no longer present.

This is the first block that I signed on the chain which recorded v0.22.1, and the last block that I signed on Steem, which shows that I accepted HF23 at a particular timestamp.

There probably are more operations that can be done Hive.

There are more than 50 different operations on Hive, which can be found here. Some of them were depreciated in previous hardforks, such as creating an account with delegation.