Sorry, I disagree. Putting discrete data, which is also clearly defined in its meaning, into a text field (which a data consumer would then need to parse!) is bad database practice. 

Besides, as I said, this approach is also used for https://wiki.openstreetmap.org/wiki/Tag:piste:type%3Dnordic#The_piste_as_relation and https://wiki.openstreetmap.org/wiki/Tag:route%3Dmtb - these are clearly established practices and in much wider use compared to the ferratas.

The fact that the ferrata proposal hasn't been voted upon doesn't make the reuse of an existing and agreed upon mapping pattern for this case "unofficial" IMHO.