It seems this warning is just telling you your network does not support Encrypted DNS (DoH or DoT) oBlivious DNS is really DoH and my router does not support it, also I dont use iCloud so I dont even know why it was turned on under Limit IP Tracking, as I would not gain any benefit from it being on (which I tested) also I was losing some throughput. I know some Asus routers with Merlins firmware support it, but since mine does not as it examines my DNS lookups to make sure I'm not going anywhere bad, a bit like NextDNS its fine for me to not worry about it and turning it off just gets rid of that warning on my home network. I have not seen the warning since, but basically its turned off so I wont but since it wasn't doing anything for me it was just annoying and slowed me down a bit, but having the warning does not mean there is a issue its just that, a warning your hardware or network provider do not support DoH It may be worth asking your router manufacture or ISP if you use their one if they plan to enable it.
Some VPN's are getting confused by it too as they think its a VPN already (it isn't as we know) so they wont work as they think you have one already, I guess this is why its still in beta. Be aware turning off Limit IP Address Tracking is like turning off Relay for your home Network or Cellular provider, but if it does not work and you get that warning, there really is no harm anyway turning it off for now as I see it.
*Edit Well that didnt stop if for long so I turned it back on, basically unless your router supports DNS over DoH or DNS over DoT this warning will keep coming back. I wish Apple would sort it out so it didn't persist as I never had this warning in 14.8.1 which had Encrypted DNS I believe and people keep restarting their phones trying to get rid of the thing until it comes back again. Most routers don't support this and corporate/Enterprise networks more than likely won't either, as they tend to monitor their environment more closely. Apple should turn the heat down a bit for the average home network because I'm not going out to buy a new router that does support it just to get rid of a warning, and I didn't have to turn off my iPhone and iPad in iOS 14 to stop this warning.