This flag is intended for standardness-only, and the primary goal is to
make is so that malicious super-high-density-SigChecks transactions cannot
get into the mempool, where they would frustrate simple block assembly
algorithms.
A transaction conforming to this rule cannot (overall) exceed a density of
1 sigcheck per 36.666 bytes.
(ref: https://gist.github.com/markblundeberg/becf85ac6ba48bebfeb7871ce2de27a7)
Depends on D4678