Manual:$wgUseKeyHeader

HTTP proxy (CDN) settings: $wgUseKeyHeader
Send the Key HTTP header for better caching.
Introduced in version:1.27.0 (Gerrit change 243895; git #7e20cb51)
Deprecated in version:1.32.0 (Gerrit change 466902; git #6258e5e5)
Removed in version:1.34.0 (Gerrit change 517919; git #ce1bd86)
Allowed values:(boolean)
Default value:false

Details

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Based on draft: https://datatracker.ietf.org/doc/draft-fielding-http-key/