Backstabber's description says nothing about it applying a slow (it mentions doing Damage Over Time, reducing their bullet resist, and giving your nearby allies bonus movespeed and vision on the target through walls).
But when I shoot an enemy with it (and nothing else), there is a very clear Slow visual (the yellow circle and yellow down arrows), and it *felt* like I was being slowed when an enemy Calico shot me with it but maybe that was a coincidence (she may have had a different slowing item).
Anyway, at minimum a visual error (unless the "slow" effect is meant to be representing some other debuff here, in which case: it's a little confusing).
But when I shoot an enemy with it (and nothing else), there is a very clear Slow visual (the yellow circle and yellow down arrows), and it *felt* like I was being slowed when an enemy Calico shot me with it but maybe that was a coincidence (she may have had a different slowing item).

Anyway, at minimum a visual error (unless the "slow" effect is meant to be representing some other debuff here, in which case: it's a little confusing).