Consider this simple query:

FROM employees | KEEP first_name | EVAL x = 1.0 | DROP x

In this case, the PruneColumns rule will remove EVAL x in the first (non-local) logical optimization.
But for this query:

FROM employees | KEEP first_name | EVAL x = 1.0 | DROP x 
| FORK (WHERE true) (WHERE true) | WHERE _fork == "fork1" | DROP _fork

It will not. The x field will eventually get dropped in the local logical optimization, but this might be too late, e.g., when performing the late materialization added in #132757.