Hi Chris,
You are incorrect, this is not a hosting issue. To test this case, I can simply turn on and off your plug-in to see the HTTP status change. Installing and turning on different plug-ins is pointless, unless they are also setting the status to 503, besides that I am not putting the site in maintenance mode, I am creating a coming soon page.
If you took a look at your code for the under construction plugin you will see that the x-under-construction > functions > output.php file has a function called x_under_construction_output
that sets the HTTP status to 503 in line 41
.
Maintenance mode is different than a coming soon page. I can see an argument being made for under construction and maintenance being the same/similar and wanting the status to be 503 so that you are not dinged for SEO and you want the bots to come back and re-index your site at a later date.
However if I am creating a coming soon page, wouldn’t i want the headers to return a 200 status so that it can start indexing that page? So that the company’s contact and business information is starting to propagate in search engines.