I overlooked the part that the areas are spread over whole Europe :frowning: and can only second @mcliquid with his detailed informations.

And yes, I think now that informal=no can make sense in special protected areas like Way: ‪Riedbruch‬ (‪1065056474‬) | OpenStreetMap that is part of the protect_class=2 area Nationalpark Hunsrück-Hochwald . But I wouldn’t use it for the regular ways of the enclosing protected area.

I may add that it can be valid to change a way to abandoned:highway=* as I have done with Way: 39497266 | OpenStreetMap after a survey and a bit of research afterwards. This way was destroyed but the former course can still be tracked.

It’s a good idea to add a note to such a changed way (not only the changeset) because otherwise it can lead to a history like this one: Way History: 148232656 | OpenStreetMap

For most protected areas in the region where I live (French-German border) this is not the case.