The main asset of photo.net is its wealth of knowledge from decades of earlier threads. I was in college when Philip Greenspan started the website and I even used its software to build our own college student community forum. When I do google search on obscure vintage camera or lens, often there are previous discussions on photo.net.
I also visit it regularly, especially the Classic Manual Camera sub-forum. It is still reasonably active with a good friendly albeit small community.
As several noted, it is a simple SSL certificate expiration issue. If it were my own site, I could have gotten it sorted out in 15 minutes. The main obstacle might be that today's admin might not be the one who did the certificate years ago, so there might be some ownership/access privilege issue.