But why would Google have indexed style.cfg, except in some case where it was explicitly asked to? There are no "links to style.cfg" that Google should be following on phpBB sites. Those style.cfg-related searches look like they are only finding sites that have accidentally or intentionally left directory listings enabled.
Separately, POWERED_BY is also subject to translation now. So any of the "Powered by phpBB" searches are only going to hit sites where translation of this phrase is still in English, even if the site does still have the copyright info displayed.
Maybe picking on a phrase like "It is currently" in addition to "Powered by phpBB" could help count "just the front page of phpBB sites." Not withstanding this literal phrase is "only English" and "only if the style in use actually displays it", too. Otherwise any of those earlier searches in this topic were counting "all phpBB-hosted pages indexed by Google"
, and not "a count of phpBB sites indexed by Google."
And repeat that for every language pack, including where it's seen the POWERED_BY phrase has or has not been translated.