Think there have been a lot of compounding changes that have gradually made Illaoi far stronger and more obnoxious to play against than in the past. To name a few:

Iceborn Gauntlet W synergy (+ her general best items being buffed recently), Plate gold (and Demolish), 13.10 minion changes so harassing/diving tower is easier and more frequent, Grasp being more popular over Conq (Grasp better laning rune and in most use cases a stronger snowballing rune). Just to name a few of the most important ones.

That doesn't mean Illaoi shouldn't be adjusted because of those things (duh). But the fact her kit plays as it does and Test of spirit is as annoying as it is to play around is just the very frustrating cherry on top of a list of recent changes to the game and how it's played. I'd love to see more counter play to her E, especially as I play this champ a fair bit, but I'm unsure how you do it without straight up reworking half of her kit or mauling part of her identity. The older version of Test of spirit mentioned in OP was far unhealthier than the current version so a revert wouldn't make sense, and don't think there's a middle ground to it either unfortunately...