Sorry but I disagree.

The use of the via_ferrata_scale key is described in the official wiki https://wiki.openstreetmap.org/wiki/Key%3Avia_ferrata_scale
Being a key, the only way to use it is the official one which clearly specifies that the key can only be used on the "way" element, see the symbols under "Used on These Elements".

The wiki page you mention has been a "proposal" type for several years, in fact, it isn't even a real proposal, given that it is in "Draft" status, as you can see:
https://wiki.openstreetmap.org/wiki/Proposal:Via_ferrata#Tagging_as_route=via_ferrata_relation.
Using the via_ferrata_scale key on a "relation" is in clear conflict with what is specified on the official page.
There cannot be different mapping ways for the same key, the only correct mapping way is the one specified on the official page.
Also on taginfo, you can see that the only reference is to the official wiki:
https://taginfo.openstreetmap.org/keys/via_ferrata_scale#wiki

If you want the key to also be used in "relation", you should propose a change to the official wiki.

Also consider that there are excellent viewers that effectively use the tag as described on the official page, including the value of via_ferrata_scale, for example this one:
https://it.mapy.cz/turisticka?q=sella%20nevea&source=osm&id=142561475&ds=2&x=13.4574869&y=46.4218843&z=16
The official mapping method works, works well and can be visualized well.