That's because there actually is a comment in /etc/resolv.conf on MacOS telling you that it isn't used by the libraries there. This is ironic, given the context here.
#
# Mac OS X Notice
#
# This file is not used by the host name and address resolution
# or the DNS query routing mechanisms used by most processes on
# this Mac OS X system.
#
# This file is automatically generated.
#
/etc/resolv.conf hasn't been the place for this for the past 17 years, this having changed in MacOS 10.3. It's in the system preferences plist now, modifiable with the networksetup tool.
No, I've never seen cleanbrowsing and it's ilk; at least in this case it is a comercial companies trying to force the system to work with their product, not a part of community supposedly "fixing" the system and breaking it in process. They also do not force resolv.conf, only using the new configuration mechanism to make their changes stick.
Unbond & co are of course not stub resolvers; however the classic stub resolvers (like glibc nss dns module) are kind of obsolete too. All modern systems have an service, that does the resolving and caching in the separate process and the nss just asks that process. Just like unbound & co. With properly configured nsswitch (e.g. with system-resolved and nss-resolve), the resolv.conf is not used either, just like with MacOS. Resolv.conf pointing to local resolver is only for processes, that ignore nsswitch and would be break otherwise.
And you've clearly never seen https://community.cleanbrowsing.org/knowledge-base/change-dn... or its ilk. (-:
I'm pointing out that unbound, dnsmasq, et al. are not "stub resolvers", by the way.