While looking at my 404s the top one for the blog was /.well-known/traffic-advice.
This is part of the traffic advice mechanism to control traffic from prefetch proxies (and based on my current access logs, seems only used by the Chrome Privacy Preserving Prefetch Proxy).

The traffic advice mechanism is specified in the document here.
It can be used to reduce the number of requests coming from prefetch proxies.

To get rid of the 404s and provide support for the traffic advice mechanism, I use the following snippet in my nginx config.
It allows all requests from prefetch proxies (as currently I see no need to limit them).

# Private Prefetch Proxy
# https://developer.chrome.com/blog/private-prefetch-proxy/
location /.well-known/traffic-advice {
        types { } default_type "application/trafficadvice+json";
        return 200 '[{"user_agent":"prefetch-proxy","fraction":1.0}]';
}