I was doing some digging. Findings so far:
HBASE-7224) added this to the javadoc for HbaseObjectWritableFor96Migration
* @deprecated This class is needed migrating TablePermissions written with
* Writables. It is needed to read old permissions written pre-0.96. This
* class is to be removed after HBase 0.96 ships since then all permissions
* will have been migrated and written with protobufs.
However, when reading the code TablePermissions, which extends Permission which is still Writable and never made PB I guess. There is also a couple of other Writables like AuthenticationKey.
It seems (from my reading of the code) that we are using PB's in the RPC, but we are reading and writing these objects from the ACL table or ZK using Writables. However, I think neither of these depends on HbaseObjectWritableFor96Migration per se, so the patch maybe good as it is.