A closed way for a boundary is not totally fine and is actively harmful.
Suppose you have a boundary relation with one outer and one inner:
relation: boundary=administrative + admin_level=8 + name=Chicago
Suppose now that the two members also repeat that tagging:
outer closed way: boundary=administrative + admin_level=8 + name=City of Chicago Boundary
inner closed way: boundary=administrative + admin_level=8 + name=Chicago Boundary
You now have, perhaps, from a data consumer perspective, the following:
- A boundary relation for Chicago
- A boundary defined as a closed way for Chicago Boundary and City of Chicago Boundary.
So where you intended to have just one boundary called “Chicago”, instead you have three boundaries with slightly different names. And worse, the inner one (called Chicago Boundary) actually surrounds an area that is not even in Chicago.
The database is absolutely littered with cases of this, and the only sane thing a data consumer can do is treat boundary relations as the sole source of city boundaries and ignore boundary tagging on member ways.
So I strongly disagree - all boundaries must be modeled as relations in order to distingush them from the massive amount of boundary tagging that is repeated on member ways and would be wrongly interpreted to be a boundary in all sorts of ways.