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This allows you to block certain versions from logging into your server, every Minecraft version has a special number assigned to it called a Protocol Version. You can view a list of them here and find a generator here. For example, if you wanted to block 1.10 Minecraft you can see from that list that the number is 210. You would then change your config to, block-protocols: [210] , you can then add as many protocols as required separated by a comma if you wish to block more.
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The default will be filled with the lowest version your proxy can handle. You can view a list of them here. The key is the BungeeCord / Velocity server name from your config.yml and the integer is the protocol version from the previous link.
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