In Mac OS Sonoma, browsers now require and the OS will ask for the “Local Network” permission to access local websites. I didn’t know why it was asking and didn’t allow it, but then couldn’t access my sites. I had to go to “System Settings”, the “Privacy & Security” pane, select “Local Network” and turn on for my browser(s) to get access again.
If it matters, my local dev setup uses domains set in /etc/hosts pointing to IPs of VMs run by VirtualBox, managed by Vagrant, set up like web.vm.network 'private_network', ip: '192.168.56.1'.