SWUs used to be pretty easy to confirm on domestic routes outside of the 3-class flagship routes (JFK-LAX, JFK-SFO, select LAX-MIA). This is because domestic first upgrades were tied to the A fare bucket, which was also a discounted First class fare and AA at the time did not have the ability to separate them out as upgrade space vs revenue space.

International upgrades, AA was better able to separate out since upgrades to Business were booked in the C bucket, which was exclusively for miles/SWU upgrades from economy. Upgrades from biz to first were still plentiful as the A bucket could not differentiate between upgrades and paid bookings.

Eventually, AA re-coded domestic first with business fare buckets instead, so even though it was branded as First, it was actually a J/D/R/I ticket with straight awards being U and upgrades being in C buckets. This gave them the ability to better control upgrades on domestic flights using instruments.

Then, AA eventually was able to better control the A bucket so that it could be closed to upgrades but open to paid revenue fares, making biz to first upgrades much tougher (but that will become irrelevant in the next year or so).