Currently, on all responses mw.config includes configuration variables wgCookiePrefix, wgCookieDomain, wgCookiePath, and wgCookieExpiration. These are documented[1] as being
@internal For mediawiki.cookie
However, since rMWa0370fe3c958: mediawiki.cookie: Export config via packageFiles by @Krinkle, mediawiki.cookie doesn't use mw.config to retrieve these settings, but rather they are injected via packageFiles. Thus, they should no longer be needed in mw.config.
Per codesearch, there are no uses in deployed javascript: https://codesearch.wmcloud.org/deployed/?q=wgCookie(Prefix|Domain|Path|Expiration)&i=nope&files=\.js$&excludeFiles=&repos=
However, a number of non-deployed extensions still use these, so an entry in the release notes is probably a good idea: https://codesearch.wmcloud.org/search/?q=wgCookie(Prefix|Domain|Path|Expiration)&i=nope&files=\.js$&excludeFiles=&repos=