i dont understand what this has to do with his stealth, nor ( as far as i know ) did it ever work on his E, as his E does not proc item effects such as rylais or liandrys ( if you could toggle his E and it costs mana i would argue that it should proc item effects but seeing as it boosts your basic attacks with bonus magic dmg which adds its own poison instead of directly doing ability damage then it should not proc items effects (would just be too OP without the need of .MANA.gement )
as for Chos E it is a toggle (should cost a small amount of mana IMO even if its just 3 mana per attack) and it isnt the basic attack applying anything, the basic attack launches magic spikes that deal ability damage on their own which in turn will apply proc'd effects
i hope this clarifies things for you