The FAs said that Delta had gotten rid of the snack boxes and are selling individual items now, but I wasn't sure if this had spread out across other (900+ mile) domestic flights yet.
These have been testing for a few months, and are currently only on the test routes of LAXATL, SEAATL, and SEALAX. Anecdotally, I've heard the test is doing well, so maybe we'll see these as a permanent option on more flights several months down the line.