Still avialable through the API. According to people that have tried both Fable nad 5.6, Fable is clearly better at coding. So i expect a lot of people to pay extra for it.
Who is going to pay API prices for using Claude? I don't know one single company. It's not gonna happen.
But if 5.6 is better than Opus, Claude Code Max plan users will switch to OpenAI Codex en masse. Using Fable at API pricing is expensive