I think it was the one that was available in chatGPT before there was that switch to turbo.
From description in chatGPT:
Legacy (GPT-3.5) [Deprecating Soon]
This is the previous Plus model. It will be deprecated on May 10th. Please switch to Default (GPT 3.5) or GPT-4.
I rather they use the resources for the actual 3.5 turbo and GPT4
They have to discontinue outdated less efficient models to make space for the new ones
They all take resources to run
it also shows lower speed in the description of the model
Which might mean it took more resources to run than the Default 3.5