This was meant to implemented but there is a missing return() for the mapply(). Will be sending a PR for a fix with tests.

https://github.com/ycphs/openxlsx/blob/master/R/openxlsx.R#L134-L146

library(openxlsx)

x <- list(withFilter = TRUE, keepNA = TRUE)
openxlsx_setOp(x)
#> Error in as.list(value): argument "value" is missing, with no default

^{Created on 2021-07-02 by the reprex package (v2.0.0)}