Actually it is not historical but quite new. It the polish first and so far only metropolitan area created in mid 2017 as an association of communities/municipalities within the Silesian Voivodeship which came into effect on 1st January 2018.
As fas as the territory is concerned it is smaller than a Voivodeship but greater than a powiat that is why I suppose the mapper changed the level to AL5…
This is Metropolitan area. This is something new in Poland, started to operate beginning of this year and for now there are legal grounds only for this one instance. As far as I see, some other countries (Brazil, Bolivia, India) decided to mark Metro areas with admin_level, while some other not (Turkey, Austria, Sweden) - at least that’s what comes from quick cross check of our Wiki and Wikipedia.
I don’t have yet an opinion about it’s this particular relation in OSM.
but there is a geometric problem: Usually admin boundaries are hirarchical organized.
E.g: All AL4 together build the AL2 (country), and all AL6 of a state (AL4) build that state.
Now you are creating boundaries, which overlap existing boundaries. e.g one AL5 contains parts of different AL4.
That is breaking the unwritten rule: “an area can’t be a partial member of two other areas”.
In my point of view the “association of communities/municipalities” is not an Admin-Boundary (boundary=administrative) but something else, e.g. boundary=region as we did in Germany.
osm-wiki says: “The admin_level key describes the administrative level of an object within a government hierarchy.”
and this AL5 is breaking the rule.
What I’m trying to point out, is that this problem might be related with having metropolitan areas tagged as administrative boundaries, which is not that common, and usually there would just a few such areas per country.
And they take some duties of local administration to a higher level that’s why I see the point in tagging it as a administrative boundary. OTOH your arguments are also valid.
What we need to get is tagging scheme for such objects, nice if it could be consistent across world.
I guess that you would need to contact all countries that do have admin_level defied for metropolitan areas.
Another counterexample is the Canadian town of Lloydminster, which has a single municipal administration while half of it is part of Alberta and the other half of Saskatchewan. No, this is not a problem in our data, source.