This is a longstanding tradition from FreeBSD. A list of our commit message trailers: https://docs.freebsd.org/en/articles/committers-guide/#_incl....
"Sponsored by:" search in FreeBSD commit messages: https://freshbsd.org/?q=%22Sponsored+by%3A%22
"Sponsored by:" search in FreeBSD commit messages: https://freshbsd.org/?q=%22Sponsored+by%3A%22
It also helps with lawsuits, which was a major thing right at the beginning of history of BSD.
* https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc.....
Linux later had the lawsuit issue with SCO and IBM and tracking where certain things came from (which was not helped by the fact that Linus Torvalds refused to use source code tracking for the longest time (later taking up Bitkeeper, and later developing git)).
You likely mean the end of the effort to replace all AT&T code, which was more than a decade later.