Current versions of Sendmail can speak LMTP to local processes, but how to make that happen seems poorly documented. When trying to get it to talk to GNU Mailman3 recently, I found next to no documentation, and a lot of people asking how to do it, so I'm leaving this here in the hopes that it helps someone.
This document shows a simplified example of how I did it on my Ubuntu 20.04 system, at least in terms of Sendmail configuration. While I'm using a "lists" subdomain, that seems entirely optional with this technique.