First of all, every ruins has a blueprint:
Core Rules page 48 wrote:FOOTPRINT
When this terrain feature is set up on the battlefield, both players must agree upon its footprint – that is, the boundary of the terrain feature at ground level. This is so that players know when a model is wholly within this terrain feature. If this terrain feature is mounted on a base, then this will define its footprint.
Secondly there is the Autumn Balance Dataslate change to the visibiliy rules:
BALANCE DATASLATE AUTUMN 2023 wrote:‘Models cannot see over or through this terrain feature (i.e. a unit outside this terrain feature cannot draw line of sight to a target on the other side of it, even if it would be possible to draw line of sight to that target through open windows, doors, etc.) Aircraft models are exceptions to this – visibility to and from such models is determined normally, even if this terrain feature is wholly in between them and the observing model. Models can see into this terrain feature normally, and models that are wholly within this terrain feature can see out of it normally. Towering models that are within this terrain feature can also see out of it normally.’
Now please review pages 11-13 of the Rules Commentary for some excellent pictures of what this means during play.
Now to answer your question, there is no Obscuring in the current rules. You either have
LOS or you don't. However, you are not allowed to draw
LOS out a Ruins you are not
wholly within. You can't touch one side of a ruins to gain
LOS past it. You have to go wholly within it. It right there on page 48 of the Core Rules, so pull it up the next time someone tries going back to 9th Edition terrain rules.
Needless to say, none of what I wrote above applies if you aren't playing 10th Edition.