I think it would be more accurate to say that 2.5" shocks are required, not the fox shocks, at least for the 19/20 version of the shock. Otherwise, other coils wouldn't be compatible with the shocks, ie Icon 158504's or Kings. The lower spring perch on the '21 has changed, which seems to limit aftermarket offerings further to only the Westcott "not-a-spacer" or the Eibach redesign forthcoming.
Correct, thanks for clarification. I believe Eibach has mentioned newer spring design for the ‘21 later this year.