I see no SLA on how long does OpenAI retain fine tuned models. We are a tier 5 company using OpenAI extensively.
What happens if our FT model was based off of gpt-3.5-turbo-0301 (oldest currently available model GPT 3.5 model) but that becomes no longer available? Will our FT model also become unavailable?
There is no defined retention period for fine-tuned models. You can delete fine-tuned models when you no longer need them.
OpenAI provides details on model deprecation including the impact of deprecations on fine-tuned models here. So far, it looks like fine-tuned models generally remain at least for some time available even after a specific base model was deprecated. Additionally, the advance notice is typically long enough to migrate to a different model.
Also, if I am not mistaken, then gpt-3.5-turbo-0613 was the first gpt-3.5-turbo model that could be fine-tuned.