Great writeup, and very interesting lessons to learn! For me, the most suprising part was the fact that a blocked service might cause an unsane increase in traffic (due to retries of badly written clients).

However, two things look a bit strange to me:

> On the day following the API termination, I received more than 1TB of incoming requests, [...] it doesn’t make sense for me to keep paying for data transfer overcharges.

This looks like the author chose a very bad hosting plan. For example, Hetzner simply lowers your connection speed if you get above the limit, and you only pay for additional traffic (and speed) if you really want to. Most other providers handle the situation in a similar way.

I know that those plans existed since at least ~15 years ago. At that time, our main criterion on a hosting provider was their traffic payment plan. We considered only providers who had a clear upper bound on the monthly costs.

> I stopped the service as planned, on November 15th, after a two weeks notice.

This seems to be very short period for closing down a service. Of course, there were no paying customers, so they got more than they paid for. However, some trouble could have been avoided by a longer closing period. In particular, it is much easier to get rid of annoying after-period users if you are able to deny their requests with a simple "You had 3 months to prepare for the shutdown!"