Manual:$wgCookieSameSite

Details
This only applies to login cookies, since the correct value for other cookies depends on what kind of cookie it is.

Addition
This variable was added in MediaWiki 1.35.0 (608993).

It was backported to 1.34 as part of the MediaWiki 1.34.3 release (612335).

It was also backported to 1.31 as part of the MediaWiki 1.31.9 release (618543).