After some experimentation, we find that you can in fact merge fields, if they
1. Are Marketo custom fields - aka: not Marketo system-generated fields.
2. Are of the same field type - no surprise here.
3. Are not in use. You would have to remove all dependencies first.
Once that is the case, you can define which field's value wins in case of conflict, and then you can merge away.
Good function. Odd though that it appears like a ghost. Field merging - at least in my experience - was always a support-only function. Also I wonder how it behave with CRM-synced fields.