I suspect it is too much traffic from specific IP addresses
When people use VPN’s or proxies, they tend to share a common IP address range, and also company or education sites where several users access the internet through the same common network connection
I think, over time, the VPN IP addresses (and cloudflare relay servers) are getting blacklisted as “high users” at ChatGPT. That would explain why it took a while to show up and is getting worse over time