pandoc/test/command/11270.md

% pandoc -f markdown -t native -s
---
header-includes: |
    \makeatletter
    \beamer@ignorenonframefalse
    \makeatother
...

^D
Pandoc
  Meta
    { unMeta =
        fromList
          [ ( "header-includes"
            , MetaBlocks
                [ RawBlock
                    (Format "tex")
                    "\\makeatletter\n\\beamer@ignorenonframefalse\n\\makeatother"
                ]
            )
          ]
    }
  []