I thought that it was iCloud private relay. I turned it off (and just checked that it is off). Now I can't reach linkedin.com. I have no problem on my iPhone 13, nor on firefox on my mac. Does anybody have any idea what's going on, or, more to the point, how to debug this? That is, can I look at some tracing to find out what the hangup is?

Ok, I found the answer: iCloud Private relay. When I turned it off safari found the site. I would ask that if iCloud private relay doesn't want to connect to a site the message should say that, rather than the site doesn't exist.

Is Safari using their own DNS? This also fails on my iPhone, but if I do the following in terminal on my iMac things resolve: dig detexify.kirelabs.org ; <<>> DiG 9.10.6 <<>> detexify.kirelabs.org ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 940 ;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 512 ;; QUESTION SECTION: ;detexify.kirelabs.org. IN A ;; ANSWER SECTION: detexify.kirelabs.org. 21600 IN CNAME amorphous-bayou-r4cweh217gotth8cf71o28yt.herokudns.com. amorphous-bayou-r4cweh217gotth8cf71o28yt.herokudns.com. 3 IN A 54.161.241.46 amorphous-bayou-r4cweh217gotth8cf71o28yt.herokudns.com. 3 IN A 52.202.168.65 amorphous-bayou-r4cweh217gotth8cf71o28yt.herokudns.com. 3 IN A 54.237.133.81 amorphous-bayou-r4cweh217gotth8cf71o28yt.herokudns.com. 3 IN A 18.205.222.128 ;; Query time: 132 msec ;; SERVER: 8.8.8.8#53(8.8.8.8) ;; WHEN: Sun Feb 13 15:54:31 EST 2022 ;; MSG SIZE rcvd: 182