In the article I linked to before, Brian Cronin says the following:
In other words, it doesn't make sense to deny you were copying one thing by claiming you were copying several other things.
And I can totally believe that someone as unoriginal as Liefeld, in the course of basing a character on several conscious influences, might also be unconsciously influenced by something else along the way. Maybe there were just so many different influences that he overlooked one in the clutter.
How popular was Deathstroke when Deadpool was created. Outside of the Titans he wasn't a big player for many years in the DCU.