Comment by dwattttt
The question of how to represent things not specified in the original format is a tough one.
At the loosest end a format can leave lots of space for new symbols, and you can just use those to represent something new. But then not everyone agrees on what the new symbol means, and worse multiple groups can use symbols to mean different things.
On the other end of the spectrum, you can be strict about the format, and not leave space for new symbols. Then to represent new things you need a new standard, and people to agree on it.
It's mostly a question of how well code can be updated and agreed upon, how strict you can require your tooling to be w.r.t. formats.
The problem is IUPAC just exists.