Update: MacOS automatically updated last night. Now https://www.youtube.com/check_content_restrictions shows moderate for all domains except YouTube-nocookie.com. There’s still objectionable content getting through, and I can still toggle the restricted mode switch in settings.
So, when I was connected to the guest Wi-Fi at my place of work, the filtering wasn’t working. When I got home, I see that YouTube is set to moderate for m.youtube.com and the APIs but not http://www.youtube.com or http://www.youtube-nocookie.com. Here are the dnsleaktest results:
IP Hostname ISP Country
188.8.131.52 dns-edge-europe-amsterdamx.cleanbrowsing.org. QuadraNet Amsterdam, Netherlands