Clearly I am not the person to give a positive answer, but...

Yes, support for Header Sources has been dwindling version by version.

However, what Header sources ever gave you, in effect, was a list of field
names and the facility to insert MERGEFIELD fields with those names into
your Mail Merge Main Document, without actually having to have any data
source or a even a particular type of data source. I suppose that if you do
not have Header Sources, you also lose the idea that users working with that
Mail Merge Main Document should /only/ be able to use the fields specified
in the Header source.

Other than that, as far as I can tell, that isn't very much different from
either
a. using your header source as an initial data source (although you might
need to put some data in your header source, e.g. by duplicating the header
row so every column has some text data)
b. setting up your mail merge main document fields using that data source
c. disconnecting the "header" source before you save the mail merge main
document, by clicking the Mailings tab, Start Mail Merge->Normal Word
Document option. You will lose any sort/filter options you set up.
d. letting the user choose the data source after they open the document.

or perhaps (a)+(b)+(d), assuming the user has a copy of the "header" source
and understands that they have to connect to the real data source instead.