I've checked all my Ford and Mercury info, and they all indicate the same thing. The '71-73 Grande, Mach 1 Sport Interior optioned Mustangs and '71-73 Cougar Vert, and XR-7 came with the trim on ALL pedals. There was a Décor option on the Cougar that included several upgraded exterior and interior items along with the pedal trim on all pedals for the interior.
Original and subsequent '73 owners seem equally split on the "Have-Don't Have" trim on the accelerator pedal. There may have been some supply issues at Dearborn. The end of the line was fast approaching for the first-generation Mustang/Cougar, and since these trims were specific to these two cars, the supplier may have already scaled back production.
Regarding the pedal trim, I only had trouble with the accelerator pedal. The gas pedal was plastic and would flex to the point the trim would pop off, and eventually, the tabs would break. I drag-raced my '71 Mach1 pretty hard, which meant there was always a size 13 foot, planting the pedal down to the point where the accelerator pedals were breaking. Unfortunately, I liked the pedal trim's look, so I bought many of the pedals and trim over time! When I received notice of them being on Ford's soon-to-be-discontinued list, I managed to stockpile several of each. The plastic pedal was discontinued on 2/80, and the trim was discontinued on 7/79. I did end up using them all. To my knowledge, I don't believe the accelerator pedal trim is being reproduced, but the others are available.