/usr/bin/registrar is in rsplib-legacy-wrappers 3.0.1-1ubuntu3.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 | #!/bin/sh -e
echo >&2 "WARNING: Usage of \"server\" is deprecated, it has been renamed to \"rspserver\"!"
rspregistrar $@
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