Hi, net-p2p/bitcoind-0.17.1 init script refers /var/run: /etc/init.d/bitcoind:BITCOIND_PIDDIR=${BITCOIND_PIDDIR:-/var/run/bitcoind} /etc/conf.d/bitcoind:#BITCOIND_PIDDIR="/var/run/bitcoind"
So? /var/run is where this kind of thing has always been?
According to FHS 3.0, /run should be used instead of /var/run. See http://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03s15.html
It also allows /var/run, and other systems (older FHS and non-FHS) use that, so it seems more compatible.