A tarpit is a service purposefully delaying network connections on a machine, typically to significantly slowing down attackers trying to send spam, and performing port scans.
One of the first public implementation of a tarpit scheme was named LaBrea, written by Tom Liston and published around 2002.
At the end of the same year, in December 2002, Theo de Raadt committed spamd to OpenBSD, offering similar functionalities.
On Linux, the xtables-addons provide tarpit abilities to iptables, but there isn’t an equivalent for its successor, nftables.
While this isn’t a super-important mitigation, it’s a pretty fun one, (un)fortunately, as of 2019 where everyone has a botnet, internet-facing tarpits are becoming irrelevant.