The Fable announcement page on the Anthropic site says this data sharing will be applied regardless of the sharing setting of the company account.
https://www.anthropic.com/news/claude-fable-5-mythos-5#a-new...
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## A new data retention policy
Finally, we’re making a change to the way we handle business
customer data for Fable 5, Mythos 5, and future models with
similar or higher capability levels. We will require 30-day
retention for all traffic on Mythos-class models, on both
first- and third-party surfaces. [...]
No it says sharing is required. If you don't change the setting on your account then you simply can't access Fable, its not like the setting is ignored. I just tried this on my account and it blocks API requests to Fable.
Interesting. When I tried switching to it yesterday from Opus 4.8 ("/model" command) it didn't complain, but I didn't actually send anything to it when I saw the cost was like 2x Opus 4.8. ie switched back
I'll try to remember to actually try it tomorrow and see what happens.