Documentation/object-format-disclaimer.txt

   1 THIS OPTION IS EXPERIMENTAL! SHA-256 support is experimental and still
   2 in an early stage.  A SHA-256 repository will in general not be able to
   3 share work with "regular" SHA-1 repositories.  It should be assumed
   4 that, e.g., Git internal file formats in relation to SHA-256
   5 repositories may change in backwards-incompatible ways.  Only use
   6 `--object-format=sha256` for testing purposes.