aachso
Indeed, the relation is missing the "via" node, so it's unclear which direction it's meant to restrict. The deleted "via" node had the location 50.7743341, 6.0570375. So the relation used to restrict turning from Halifaxstraße to Valkenburger Straße and back to Halifaxstraße.

a) There's no point in doing that because Halifaxstraße is two one-way roadways, so you could just do the u-turn from one roadway to the other if you wanted to change the direction.

b) I can't see any signs on Mapillary restricting you from doing u-turns on this intersection from any given direction

I have therefore deleted this restriction: https://www.openstreetmap.org/changeset/111454930

To be fair, some of the Mapillary imagery is older than the turn restriction, but I doubt this u-turn was ever explicitly prohibited. If anyone has more recent knowledge please let us know.