Manual:$wgUseSameSiteLegacyCookies

Details
If true, when a cross-site cookie with   is sent, a legacy cookie with an "ss0" prefix will also be sent, without  .

This is a workaround for broken behaviour in Chrome 51-66 and similar browsers.

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).