ListServ replacements

What are the latest and greatest listserv tools that you use?

I have a client who currently uses a proprietary listserv called Lyris that uses Microsoft SQL Server to store all data. It was recently bought out, and the parent company will jack up its pricing to something beyond my client's budget.

Basic requirements include self-subscribe/unsubscribe/pause, automatic un/subscribe when un/hired to a specific team, or at least provide an API to do so.

Discourse or Zulip are my first thoughts, as they might support data migration.

We set up Listmonk for a client. It's open source written in Go, stores data in Postgres, has a REST API.

I didn't see any discussion feature, and it looks like it only does one-way or distribution mailing lists.

Oh indeed, that sounds pretty definitive:

Google Groups do this, mostly, I think.