Just use a site block using http:// which will match all hostnames.
http:// {
respond "No redirect"
}
Since v2.4.0, this will not prevent HTTP->HTTPS redirects for domains explicitly configured in Caddy, but this will prevent HTTP->HTTPS redirects for on-demand domains that Caddy does have certificates for.
In general, to turn off all HTTP->HTTPS redirects, you can use this global option:
{
auto_https disable_redirects
}