No Assassin Strike in AE.
There is Ninjitsu, which has a damage bonus from hide. That might of been what you saw before.
Freeedom
H: Victra Defile (Rad/Dark) & Hot Hits (Fire/SD)
V: Spike Cream (NW) & Crescent Wind (Nin/Storm)
Sub Hero Inc ARC: 1541
--If you don't like what I have to say, it was probably a typo you are refering to.
Indeed. Custom critters never had an assassin strike to begin with. They do, however, get criticals from hide.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
Massive criticals.
Painful criticals.
"I wish my Stalker could take down a Granite Tanker in one blow, too!" criticals...
My Stories
Look at that. A full-grown woman pulling off pigtails. Her crazy is off the charts.
Depending on how long ago it was (you said "a month or so," but maybe it was longer?), you may be remembering the bugged massive damage that custom Ninjitsu mobs did when Issue 14 was first released.
Oh yeah, I remember that. That was cool. 
I wish we had a way to turn that back on for select critters. With the right powerset/difficulty combinations, it made for true glass cannon foes and awesomely hectic 'must take them out before they take me out' fights. 
Yeah, I know I'm crazy. Doesn't chance that I thought it was fun when used right. 
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
I tested a friends mod a month or so ago. One of his custom characters was a stalker, it made for some fun and challenging situations.
I went to run it with one of our coalition mates, and there were no assassin strikes whatsoever. The team size was slightly smaller, but the level of the players and difficulty were the same.
I did a search, but couldn't find a record of the attack being removed from AE missions. Was it? Or was the team size at fault?