BNETDocs
S>C 0x23 SID_WRITECOOKIE
Message Id:0x23
Message Name:SID_WRITECOOKIE
Direction:Server to Client
Used By:Diablo Retail, Diablo Shareware
Starcraft Japanese, Starcraft Broodwar
Starcraft Shareware, Starcraft Original
Warcraft II BNE
Format:

(UINT32) unknown/unparsed -- Flags, Request ID?
(UINT32) unknown/unparsed -- Timestamp?
(STRING) Registry key name
(STRING) Registry key value

Remarks

Much like a website cookie, simply stores some arbitrary string to a 'cookie jar' to save preferences et al. which can be retrieved later by the server.

Not used because it was quickly discovered that storing preferences produces less problems and were faster by storing them server-side, associating them with the account. It is somewhat curious that these packet IDs are close to SID_READUSERDATA/SID_WRITEUSERDATA (0x26 & 0x27).

Found at: battle!190216FBh and battle!1901D660h, respectively.

Comments
xpeh

It's not defunct, Battle.Net clients still support it. So it can be used with Battle.Net emulators, or if someone uses man in the middle.