The base system httpd, a forked Apache 1.3, has been removed in favor of nginx(8) as the system HTTP(s) server. Apache 1.3 is available as a package, www/apache-httpd-openbsd if you need apache specific features. Otherwise, plan a conversion to nginx(8) (or wait for the new relayd(8)-spawned httpd(8)).
The upgrade FAQ has the full list of files you need to remove or replace.