Any update on this please? I don't want to have to use ACF as well as Types to achieve this, and I don't want to have to create an intermediary object as per your documentation on many-to-many, that just seems data bloat IMHO.
The feature is being considered seriously. The development team also think that it's important and useful. But is not easy to implement.
I think it will be added. But when exactly our developers will add a new feature or not is not up-to us to determine. Each issue reported in, whether a bug or a new feature request, is taken seriously into consideration. It is given a high or low priority based on a number of factors. Factors which those with the overview of all things related are in the best position to determine.
Consider this still unresolved. (The cleanup robot emailed me to see if I needed help - the answer is yes).
Do we really need to install ACF to have a custom field that is an array of ID's for a specific post type? it doesn't seem that tricky. There's also hidden link but ideally this should be within the Toolset package.