In some old manuals I see
net.inet.carp.arpbalance sysctl, that turns on load balancing between CARP'ed interfaces. Recent manuals (FreeBSD 10+) say nothing about it.
Where did that feature go and what was the reason for it?
This feature was removed in FreeBSD 10, see commit https://svnweb.freebsd.org/base?view=revision&revision=228571 . The carp system was basically rewritten from scratch. As for "why", I can only guess that it did not seem an important feature at the time of rewriting.