As suggested by TWT, I did a tracert (or traceroute on Linux).
It times out at what I determined (see below) to be the place which hosts shorpy.
I then did the same through my neighbor's WiFi, which works with shorpy. It got to
Shorpy Historical Photo Archive | Vintage Fine Art Prints and the last step before shorpy was the place that it timed out on the tracert from my own PC / router / modem.
I conclude that I have been declared a personna non grata by shorpy and it won't deal with me - as represented by my cable modem.
I wonder whether "representations" to shorpy made through the route which works would have positive or negative results.
TWT, thanks for the suggestion.
seacue
Edited to add: My next step will be to try to get a new IP address for my modem. Since I have a dynamic IP address, getting a new IP lease may accomplish this.
Further edit: But it didn't get a new IP address or lease expiration.