@everysummertime If a connection attempt is already filtered at the DNS level by the DNS service provider, you won’t see these attempts in Network Monitor. So you might get the wrong impression that an app never made any of these connection attempts.
In contrast, the blocklist functionality of Little Snitch lets you see these blocked attempts, and also re-enable some of them, if necessary.
@everysummertime Having two blocking mechanisms in parallel, but only some of the blocked connections showing up in Little Snitch caused quite a bit of confusion. It could happen that a connection was silently blocked via DNS, and users were wondering "why can’t I connect to that server, although LS doesn’t show this connection as blocked".
That said, if you really want to let Quad9 also filter your connections, you can create a custom DNS encryption setting that uses this filtering service.