Evidently he can. Remember many of the monsters have had their mLvls rewritten. Because when an item drops, item_Lvl=monster_Lvl, the iLvl of items shenk can drop have also changed. I believe shenk is level 83 now, as his area is level 80, making iLvl items he drops, level 83.
aLvl is then calculated. I get lost in the affix_Lvl calculation - i keep coming up with an aLvl of 69 - which would not allow the perstilence affix to spawn. The minimum level requirement for perstilence prefix is 80, so i must be calculating aLvl incorrectly, as he appears to be able to spawn that affix.
Shenk drops ilvl 83 items, and so the alvl of an SC from him will be 69.
He can't drop Pestilent SCs.
However, he can drop Toxic (alvl 63) of Anthrax (alvl 44).
Toxic does 100 damage over 5, or 20 per second for 5 seconds.
Anthrax does 50 damage over 6, or 8.3 per second for 6 seconds.
When poison mods combine, they add both rates and durations (which is very nice of them because it really blows the damage out).
So a Toxic of Anthrax will do 28.3 per second for 11 seconds, or 313 over 11.
Pestilent of Anthrax, which indeed Shenk can't drop, does (an outrageous) 451 over 12.
None of this (besides Shenk's mlvl) has changed from 1.09. However, in 1.09 there was a display bug that capped all durations at 10 seconds, and a Toxic of Anthrax would only be shown as 284 over 10, even though it was the same as the one you found.