VIP display page when all servers in pool marked a down, effectively downing the VIP

If I may add, there's a few options:

1. You can use your own HTML content in an iFile

   https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-implementations-11-5-0/27.html?sr=47873155

   when HTTP_REQUEST {
       if { [active_members [LB::server pool]] < 1 } {
           set ifileContent [ifile get "/Common/iFile-index.html"]
           HTTP::respond 200 content $ifileContent
           unset ifileContent
       }
   }
   

2. You can simply respond with some static HTML

   when HTTP_REQUEST {
       if { [active_members [LB::server pool]] < 1 } {
           HTTP::respond 200 content "put your html content here"
       }
   }
   

3. You can issue a redirect to an external page

   when HTTP_REQUEST {
       if { [active_members [LB::server pool]] < 1 } {
           HTTP::respond 302 Location "http://notfound.domain.com"
       }
   }
   

4. Or as Nathan suggests you can simply use the fallback host option in the HTTP profile to force a redirect to an external site.

In all cases though you need a reasonably good monitor applied to the pool.