In the mountains you often find old abandoned buildings. The level of abandonment may vary, but many of these structures are not more than piles of stones in a suspiciously straight, orthogonal shape. I’m talking about ruins here. How do we tag these? The ruins key is not appropriate as it should be used to indicate “ruins that are of historic importance,” according to the wiki. These buildings are often old farm dwellings or abandoned shelters and hold very little historic importance. The building=ruins tag is described to be for structures that “are built as ruins,” i.e. purposely built ruins. What tags should we use for these buildings then? I refuse to leave them unmapped as they can be useful to hikers seeking minimal shelter or simply as navigational landmarks.
You can use Lifecycle Prefix.
Which wiki page? Looking at taginfo where ruins is used https://taginfo.openstreetmap.org/search?q=ruins and https://taginfo.openstreetmap.org/search?q=ruins#values there are a few key/value combinations that you could be referring to.
Generally speaking, the wiki is supposed to describe how people map, not “tell them that they are doing it wrong”. Unfortunately there are wiki editors who do do that. That said, there is a logic for the tagging described on https://wiki.openstreetmap.org/wiki/Tag%3Abuilding%3Druins (based on the way that the “building” tag is used elsewhere) so the advice there (don’t just use it for any old ruined building) does make some sort of sense - but it doesn’t match usage by mappers.
Given all that and based on https://taginfo.openstreetmap.org/keys/ruins#values I’d suggest “ruins=yes”.