Just by reading papers, so I might be wrong here regarding adoption among them.

My last commercial Smalltalk was Smalltalk/V, just before Java got released, so not much I can state here.

I think it was added originally in Squeak[0], Pharo got it from there.

[0] https://wiki.squeak.org/squeak/3059

It isn't obvious to me that Squeak classes have been re-organised to actually use traits?

Where-as for some reason I seem to imagine Pharo classes have been re-organized to use traits?