Missing o1 and o1 Pro Mode which were huge leaps as I remember it too. That's when I started being able to basically generate some blackbox functions where I understand the input and outputs myself, but not the internals of the functions, particularly for math-heavy stuff within gamedev. Before o1 it was kind of a hit and miss in most cases.

I agree. They are definitely trying to make the jump between 4o and 5 seem bigger, though most users never actually used o models, so maybe from a layman's perspective it was right to skip o1/o3/o4-mini?