If a Hero uses a dash ability that can go over terrain, e.g. Artanis – Blade Dash (Q), Hanzo – Natural Agility (Trait), then that Hero becomes immune to the knockback effect of Junkrat’s Concussion Mine (W) while they are dashing.
I confirmed this with E.T.C.’s Crowd Surfer (level 4) talent. E.T.C. can normally be knocked back with Concussion Mine while using Powerslide (Q), but becomes immune to the knockback after taking the Crowd Surfer talent, which let’s Powerslide go over terrain.