“However, beta season stats will only be available for a limited time and will not count toward a player’s permanent Competitive Season stats.” from Introducing Role Queue - News - Overwatch. People have generally interpreted this to mean that nothing, including MMR changes, from beta season will be kept. I’m not 100% convinced. As a scientist, why throw away all that data? As an engineer, maybe there is some technical reason that makes keeping the stats difficult.
Edit, Scott just answered this (MMR is kept):