By essence, anyone in any country in the world can use MetaMask.
MetaMask is a self-hosted wallet that interacts with Ethereum and other blockchains through nodes, the most popular of which are run as a service by Infura. As a matter of convenience, MetaMask uses Infura nodes by default to read and transact on the Ethereum blockchain. MetaMask is distributed for free and designed to be customized, so a user can decide to use Ethereum nodes other than those operated by Infura, or nodes relating to entirely different blockchains such as Binance Smart Chain and the Polygon network. It is entirely up to the MetaMask user.
Infura and all other US-based blockchain service providers are required to implement US sanctions. Infura closely monitors changes to US sanctions programs announced by the Office of Foreign Assets Control and narrowly tailors its internal controls to comply with the law. Currently, those regions are Iran, North Korea, Cuba, Syria, and the Crimea, Donetsk, and Luhansk regions of Ukraine.