> Many low level things are basically packed C-structs with this type of protocol.

Not really: C structs notably don't have variable-length fields, but ISO 8583 very much does.

To add insult to injury, it does not offer a standard way to determine field lengths. This means that in order to ignore a given field, you'll need to be able to parse it (at least at the highest level)!

Even ASN.1, not exactly the easiest format to deal with, is one step up from that (in a TLV structure, you can always skip unknown types by just skipping "length" bytes).