The model picker descriptions for Haiku were reorganized internally. Both Haiku 3.5 and Haiku 4.5 remain available; the user-facing text ("Haiku 4.5 · Fastest for quick answers" and "Haiku 3.5 for simple tasks") is unchanged.
Evidence
Model option functions at lines ~530549 and ~530558 (search for "Fastest for quick answers")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
