The syntax change was announced in Dec last year:
https://lists.gnu.org/archive/html/emacs-orgmode/2015-12/msg00525.html

It is now

#+BEGIN_EXPORT backend
#+END_EXPORT

On Monday, February 8, 2016, Oleh Krehel <ohwoeowho@gmail.com> wrote:

>
> At the current git master, the org source:
>
>     #+BEGIN_TEXINFO
>     @quotation
>     Permission is granted to copy, distribute and/or modify this document
>     under the terms of the GNU Free Documentation License, Version 1.3 or
>     any later version published by the Free Software Foundation; with no
>     Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
>     and with the Back-Cover Texts as in (a) below.  A copy of the license
>     is included in the section entitled ``GNU Free Documentation License.''
>
>     (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
>     modify this GNU manual.''
>     @end quotation
>     #+END_TEXINFO
>
> results in the following .texi output:
>
>     @TEXINFO
>     @@quotation
>     Permission is granted to copy, distribute and/or modify this document
>     under the terms of the GNU Free Documentation License, Version 1.3 or
>     any later version published by the Free Software Foundation; with no
>     Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
>     and with the Back-Cover Texts as in (a) below.  A copy of the license
>     is included in the section entitled ``GNU Free Documentation License.''
>
>     (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
>     modify this GNU manual.''
>     @@end quotation
>     @end TEXINFO
>
> This was done with `emacs -Q', no extra code except (require 'ox-texinfo).
>
> Note that the above org source works fine with the built-in Org-mode:
>
>     @quotation
>     Permission is granted to copy, distribute and/or modify this document
>     under the terms of the GNU Free Documentation License, Version 1.3 or
>     any later version published by the Free Software Foundation; with no
>     Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
>     and with the Back-Cover Texts as in (a) below.  A copy of the license
>     is included in the section entitled ``GNU Free Documentation License.''
>
>     (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
>     modify this GNU manual.''
>     @end quotation
>
>     @c Emacs 25.0.50.4 (Org mode 8.2.10)
>
> regards,
> Oleh
>
>

-- 

--
Kaushal Modi