Bone wall is not a stackable card or else you would play multiple bone walls at once
Sure, but that wouldn't make the card much stronger anyway. So I see no reason why it shouldn't be a stackable card if it otherwise works as one.
Besides, if it's counters, the the card should get destroyed by deflags.
Current version works according to this text (yes, the text is not on the card, you have to guess for it like usual, but anyway...), which is extremely complicated for something not even written on the card, but matches the "counters" version :
"
Shield.
Whenever Bone Wall comes into play, if you played it from your hand, put 7 bone counters on it.
Whenever Bone Wall would be destroyed, instead, remove a bone counter from it.
Whenever Bone Wall would be stolen, create a copy of Bone Wall under the control of the opponent with one bone counter on it, and remove a bone counter from Bone Wall.
Whenever there are no counters on Bone Wall, it is destroyed.
Whenever a creature is killed, put two bone counters on Bone Wall.
Whenever you'd take damage from an attacking creature, remove a bone counter from Bone Wall and prevent that damage.
"
(Yes, that's SIX different triggered and/or replacement abilities, if I were to speak in MTG terms.)
If it would stack, it would be that much simpler :
"
Shield.Stacking.
When you play Bone Wall, put 6 additional copies of it into play.
Whenever a creature is killed, put two additional copies of Bone Wall into play, and this ability cannot activate more than once per creature killed.
Whenever you'd take damage from an attacking creature, sacrifice a Bone Wall and prevent that damage.
"
THREE abilities fewer! With the only actual difference of being able to play two in a row and...why is that bad? I can play it next turn after the previous one is destroyed anyway. Only time it matters if I'm that low on life to be unable to survive a turn without two of these but in those situations, it is rarely much of a help anyway...still gonna lose a turn later.