I have performed a bootstrapping with 2.000 resamples of the Lee Carter model for mortality projection. The question is not specific for mortality studies, but on more general d
It looks like you need the apply family of functions. Your data is not reproducible, so I can't confirm this will work, but if you do:
apply
result <- sapply(JA_lc_fitM_boot1[["bootParameters"]], function(var) var[["bx"]][[70]])
You should get what you're looking for.