Apple has released macOS Big Sur 11.3.1 alongside iOS 14.5.1. Both were released due to security fixes but we don’t know many details.
macOS 11.3.1
You can download this update via System Preferences > Software Update. There are no release notes for this update other than the generic:
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macOS Big Sur 11.3.1 provides important security updates and is recommended for all users.
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