Question:
 
It says in Deut12:21 "…you may slaughter from your cattle and your flocks
that the Creator has given you, as I have commanded you, and you may eat it in
your cities…".  The question is:  This verse explicitly says that animal slaughter
shall be done in the manner "as I have commanded you", yet the commands of proper
animal slaughter can not be found in all of scripture.  Such commands do not exist
in the written scriptures.  The argument therefore concludes:  This verse is
making reference to a set of non-written, oral  commands given along with the
written Old Testament which enable this and similar versus to be understood
and applied. 
 
Answer:
 
I agree with your conclusion, but I do not agree with your reasoning to reach
this conclusion.  Let me explain.  The writer of Deuteronomy says that the Jews
should slaughter thier cattle or other animals as he had commanded them.  You
say that the scripture does not specify how to slaughter, but I do not agree with
this.  In fact, just a verse or two below God gives specific instructions about
how to slaughter animals.  In verse 23-24 he tells them that they must not eat
the blood.  In addition, they must slaughter the cattle in such a way that the
blood drains out on the ground.  Here is a specific command as to how to slaughter. 
They were not to club the animals to death or to strangle them, but to kill
them in a way which drains the blood.  I am assuming that they were to cut them
at the neck.
 
Nevertheless, I believe that there almost certainly were other oral commands
given to the Jews.  As an analogy there were also commands which the apostles
passed on to the early church which were not included in the New Testament. 
We know that Paul wrote some letters which are not included in the New Testament.
 I assume that this is because some of the instructions were expedient for the
time and were not intended as commands for all time.  Actually, except for the
Ten Commandments, all the laws were given "orally" to Moses, although we can assume
that many or most of them were written down in the form we now have, including
the laws in Leviticus and in Deuteronomy.  So, I agree with your thesis that
it is very likely that God spoke commands to the Jews at Sinai which were not written
down.  However, I do not believe that the particular example you give provides
support for this conclusion.  I hope this response makes sense to you.
 
John Oakes, PhD