[ibash]

I tried to find out why.

Unfortunately the change that introduces it predates the official release by a few months. And predates the mailing list by about a year:

https://trac.nginx.org/nginx/changeset/5238e93961a189c13eeff...

[chrsig]

ok, I found a commit in 2005, coming about because linuxthreads was interfering with the SIGUSR1 signal.

It looks like they wound up making it platform specific, so BSDs and unix like operating systems might still use SIGUSR1.

https://github.com/apache/httpd/commit/395896ae8d19bbea10f82...