There was a bug in some old versions of BIND caused interoperability problems
with other implementations when using SHA* TSIG keys. This was fixed in 9.7,
and in addition to correcting the key generation, we also added the isc-hmac-fixup
tool to convert older keys so they'd work properly.

That was eight years ago and the buggy releases are long past EOL. What
say we remove isc-hmac-fixup now?