https://www.iso.org/standard/77977.html
Anyone interested in doing an independent implementation can use this specification (or a recent draft of the upcoming 2nd edition). Alternatively, you can look at the source code of one of the three JPEG XL implementations currently available: libjxl, J40, and jxlatte. These are all open-source, and in a way that's an executable specification.
Is it simply just one that cost money to access?
If I made a language standard on my own it would not be a open standard from quite a few orgs point of view. It is annoying that you are just throwing around vague use of words without giving a clear definition.
What is a closed standard?
Also the world run on those documents with little to no relevance, no matter how much people dislike how you need to pay to view them. It is a sad state of affairs.
If you actually wanted to make changes you would lobby for it, you are just saying they don't exist which is of little help to change the status quo.