According to this Ask Ubuntu answer "httpd" and "apache2" are basically the same in "big" OS-es.
httpd is the same as apache2. It depends on the OS you use. For example in RHEL 6.2 it is called httpd and in Ubuntu it is called apache2.
The answer is from 2013 but it seems to me it's still valid.
Comparing to Unix & Linux SE: they have "apache-httpd" tag and relatively marginal "lighttpd", "openbsd-httpd" and "uhttpd" tags; no "httpd" tag at all.
There are questions tagged "httpd" that do not mention Apache. Most likely few of them will need to be retagged if we decide to synonymize the tags. I can review them, preferably before the synonym is approved. Should I also change all "valid" httpd tags to apache-http-server then? or will they be remapped automatically when these tags get synonimized?