Must be zero (MBZ) is a standard convention in networking, and frequently in API design for things that have no current use but might do in the future. It is because it has no meaning that it must be 0.
You might think that doesn't make much sense. Howevr, suppose I invent IPv4+ (unlikely), and persuade IANA to allocate me that bit (fantastically unlikely). The standard can then be amended to say: set to 0 for an IPv4 packet, set to 1 for an IPv4+ packet. If people have been randomly setting it to 0 or 1 before then, I have made an extension that isn't backward compatible. It would be if people had been setting it to 0 as required.
It's very unlikely that this flag would ever be used in IPv4 but it's much better to have a spare flag and not use it, than not have one and want it.
For an idea on how it works in practice, you can look at the DNS extension (https://tools.ietf.org/html/rfc2671), which defined previously MBZ fields in the DNS header.
As you asked what would happen, the answer is probably 'ignore bit and carry on'. You might find thant some firewalls might just drop the packet though. It is the Evil bit, after all.