Is it mentioned in document or in OTN site somewhere?
Since AL 2.0 it is. See section [url http://docs.oracle.com/cd/E37099_01/doc/doc.20/e25066/install.htm#CHDHDAIC]Running in Standalone Mode of the Installation and Configuration Guide.
Aside from the official support, the embedded JEE container used to enable the Standalone Mode lacks some features you possibly need for productive use, which are usually offered by "regular" JEE containers (and are definetly offered by the ones officially supported). If you don't need these features and you don't need official support, you can (technically) run a (small) site in Standalone Mode without issues.