Stated in the self-serve MSA. A Braintrust blog post says the same for its Loop assistant.
Braintrust agrees that it will not use any Customer Data to train, develop, or improve artificial intelligence or machine learning models, algorithms, or systems.
= US or EU (Pro); self-host for others (Neutral / n/a)
Pro customers choose a US or EU data plane; control-plane metadata may go to the US. No APAC region; self-hosting or BYOC is the route for other locations.
Pro Plan Customers may elect to store the Data Plane in the US or the EU.