How to initialise ReadOnlyDictionary?

If you don’t mind having an IReadOnlyDictionary instead of a ReadOnlyDictionary, you could use this, since Dictionary implements IReadOnlyDictionary:

private static IReadOnlyDictionary<string, byte> _validRevisions
    = new Dictionary<string, byte>
       {
           { "1.0", 0x00 },
           { "1.1", 0x01 },
           { "1.2", 0x02 },
           { "1.3", 0x03 }
        };

public static IReadOnlyDictionary<string, byte> ValidRevisions => _validRevisions;