@RedBeardBrian - I have mentioned to Keap several times about this issue, but they have a poor excuse in not wanting to implement a solution.
There stance is that it is a performance issue. So for nearly 20 years it was okay to do it via the XML-RPC without a restriction put into place. Oddly you can manually create a Saved Search to search on a particular Custom Field and run it via REST. Saved Searches are worse for performance because you can create complex queries.
What is even more funny, there will be endpoints to add / edit the custom fields for the various areas, but without the querying capabilities.
I have posted workaround solutions in this thread below.
If more people complain about this issue then hopefully Keap will do something about it.