The openssh site says this was depreciated with version 3. In FreeBSD this required for much longer. At lease by 12.0, OpenSSH_7.8p1, OpenSSL 1.1.1b-freebsd authorized_keys2 will not work. No error message is given.
man page: if this option is not specified, the default is
~/.ssh/authorized_keys and ~/.ssh/authorized_keys2.
authorized_keys2 should be removed.
The man page is correct - if not set by a config file the default is to check both .ssh/authorized_keys and .ssh/authorized_keys2.
The stock sshd_config (upstream and in FreeBSD) contains:
# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
# but this is overridden so installations will only check .ssh/authorized_keys