Manual:$wgApiFrameOptions

Details
Disallow framing of API pages directly, by setting the X-Frame-Options header.

Since the API returns CSRF tokens, allowing the results to be framed can compromise your user's account security.

Options are:

Do not allow framing. This is recommended for most wikis. Allow framing by pages on the same domain. Allow all framing.
 * 'DENY':
 * 'SAMEORIGIN':
 * false: