I found the culprit thanks to furas' suggestion.

def minify(self, content, minify_css, minify_js):
    return minify_html.minify(
        content,
        do_not_minify_doctype=True,
        keep_spaces_between_attributes=True,
        minify_css=minify_css,
        minify_js=minify_js,
    )

It's located in pelican\plugins\minify\minify.py