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...
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...