how did BSD libc make binary compatibility promise beetween freebsd version?

[mokhi]

If you mean using libc/libc++ API in FreeBSD with no break, This is
what FreeBSD provides by symbol versioning to provide same/compatible
API in different versions.

This means If you have a program that uses an API [printf(3) for
example] on FreeBSD 9, and a change made on that API in FreeBSD 10,
you can be sure that change is compatible and doesn't break what you
had expected till now.
This is being implemented using symbol versioning.

I'm not sure that i understand your question correct :)
This is my answer to my interpretation of your question ;) [and also
based on my interpretation of FreeBSD's behaviors and codes :D]

Also as a side-note i guess freebsd-current@ is more helpful for you
question's answer.

Best regards, Mokhi.