Yes, crossing:markings=yes was approved for “An unspecified or unknown crossing marking. Can also be used in document any crossing marking which hasn’t been documented.”
In most of the cases I’ve seen in London, where we’ve generally got very good Bing aerial imagery, the markings are very clear unless there’s tree cover. The user “upgrading” the tag at iD’s suggestion could easily have added useful information instead of just clicking the spanner icon to make the yellow warning box go away. At least, they could have done if they were presented with some options other than a suggestion of crossing:markings=yes