It seems to be: https://github.com/gohugoio/hugo/issues/3983
Documented here:
If it doesn’t work, the documentation should remove it until it does, or specify the limitations.
Note, I’m not looking for issues. The feature failed me when I tried to use it, and I wish the documentation didn’t advertise it without any caveats. I can make a pull request to add some note there for the time being, but I’d rather get feedback first.
I have found a workaround. Up to now I had used a custom template to replicate the grouping behavior, but I found out that one can override the name of frontmatter date field that Hugo looks for in pages.
This way, the regular .GroupByDate becomes viable, even with custom date fields. Since you can set more than one alternative name, I suspect this should cover many cases.
Doesn’t change that .GroupByParamDate is currently broken (I filed a pull request for the docs), but it’s a workaround.