I think everyone may be overreacting a little to how much Twin is getting nerfed. If you straight up go -4 Twin +4 Kiki-Jiki, the deck is still probably Tier 1.5, because it plays similarly aside from losing the turn 4 win (which is definitely huge).
A big point about this change is that it makes lightning bolt a viable answer against the combo regardless of the Twin/Copy target; where lightning bolt was blank against Exarch/Twin lines, Exarch/Kiki-Jiki can still be disrupted.
As a modern player, I'm very happy with these bans. I agree with the commenters here that Twin wasn't oppressive from a power level perspective, but it was pretty stifling from a diversity standpoint (I often ran into the question "why not just run twin?" and rarely had a compelling answer). Additionally, losing to twin often just felt bad because the window for interaction was so brief.
Bad feelings abound today, but I'm willing to bet that modern is in a better place following these changes.
I do like Jace in the deck, but there's definitely a tension between all the delve threats and jace for what should be cast on t2 (majors avoids this by cutting delve threats to just the two Tasigurs). I'm currently testing the list in OP minus 2 angler, minus 1 terminate in favor of 3 flip Jace. I'll let you know how it goes.
A big point about this change is that it makes lightning bolt a viable answer against the combo regardless of the Twin/Copy target; where lightning bolt was blank against Exarch/Twin lines, Exarch/Kiki-Jiki can still be disrupted.
Bad feelings abound today, but I'm willing to bet that modern is in a better place following these changes.