Vexing Shusher + illegal targets

[Barook]

What exactly happens when a target becomes illegal? I know the spell is countered and stuff, but could the spell still resolve when made uncounterable via Shusher?

E.g. I StP a MoR, she gives herself protection from white. Now I make my StP uncounterable via Shusher. What's happening?

[Pinder]

Nope. Shusher (and other cards like it) say 'by spells or abilities'. Nothing in there about state-based effects.

[Anusien]

Nope. Shusher (and other cards like it) say 'by spells or abilities'. Nothing in there about state-based effects.
This is not a state-based effect. SBEs are listed in rule 420.5a-420.5k. This is just part of the check on the resolution of a spell.

But a spell being countered on resolution for lack of a legal target is not a spell nor an ability and will not be stopped by Shusher.