Why don't you support...

A list of ideas and why Gatherers4j does not currently support them.

firstBy(fn)

Because this can be done with existing functions:

Stream.of("A", "B")
    .filter(it -> it.length() == 1)
    .findFirst()
// Optional["A"]

lastBy(fn)

Because this can be done with existing functions:

Stream.of("A", "B")
    .filter(it -> it.length() == 1)
    .gather(Gatherers4j.last(1))
    .toList()
// ["B"]