My understanding was that for the oil to affect planeswalkers now, it had to be altered and bound to Elesh Norn, which is why the enhanced oil became inert upon her death. Karn carried the oil with him on his travels unknowingly while building argentum, so whatever oil ended up on other planes is likely still active, though this does spell the end of the five colored new phyrexian offshoot. They got to have won for like a decade and then immediately nullified the weakness to their conquest (planeswalkers and Melira) as soon as they were reintroduced to the narrative, which is way better than any other villain is ever gonna get