From https://github.com/phpbb/phpbb3/pull/973 :

what about the other three?

phpBB/styles/prosilver/template/overall_footer.html:

{CREDIT_LINE}
phpBB/styles/prosilver/template/simple_footer.html: {CREDIT_LINE}

phpBB/styles/subsilver2/template/overall_footer.html:

{CREDIT_LINE}
phpBB/styles/subsilver2/template/simple_footer.html: {CREDIT_LINE}