The project history goes back to 1982. There may have been rewrites in the later 80s, but it's some of the oldest C around, and a very conservative codebase (e.g. Linux kernel gets much more aggressive refactorings regularly).

That particular git repository has history imported from 1996 onward, but Postgres was a very established project by then: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit...

These days people might not blink an eye at gofmt/rustfmt rewriting the AST to clean it up, but those toolchains were built that way largley because automating anything about large C codebases is so hard.