Poison used to be largely ignored damage type and playstyle before Anniversary. Because it didn't scale with stats and for skill points economy. If you specialized on other damage type then adding poison required considerable point investment, and these points could be spent to make your primary type better. While at the same time poison was weak on its own without several gear slots dedicated to it.
Right now with how many skill points all the builds get, rogues can afford to use 2-3 damage types efficiently, those within their mastery or others. You don't have to focus on poison/bleed fully too, just get pierce or physical and add poison to it and you're good. Not 'viable' as people call it, but very good and solid. Bows too. And casters with elemental spells, though more complicated going both dexterity and intelligence, should be feasible.
(https://i.postimg.cc/mgps01XV/corsair65.jpg)
Thats a corsair with a common ADCTH sword, Shen-Nong, monkey king, envenom weapon and throwing knives. Basically a melee build with AOE ranged spell on demand. TitanCalc (https://www.kirmiziperfect.com/titancalc-anniversary-edition/TitanCalcde3ede3ede3e.html?mastery=Corsair&master1=6&master2=5&sa=30&m1=32-8-6-12-12-1-8-0-6-0-0-0-12-0-12-1-8-0-8-6-0-0&m2=32-0-12-0-6-1-6-8-6-6-6-1-6-0-6-8-3-1-1-4-6-0)