It's retiring them, since there will be more models to choose from by then. Currently you feel that GPT-5.5 has advantages over all newer models (it asks more and therefore uses less tokens to solve a task). Within the next days the additional GPT-6 models come out. They won't ask as much as GPT-5.5 (but maybe you could also just add this to the default prompt). But cheaper models of GPT-6 will actually be better than GPT-5.5.