Currently using postfix + dovecot + rspamd on Debian, but will be migrating to NixOS-mailserver (mostly because I am migrating to NixOS anyway; it's the exact same stack under the hood, though).

Regarding self-hosting dying: yes and no. I use a relay for some of my outgoing mail, because I have to communicate with people behind allowlists, and I can't afford to get myself on one. I do not send much mail, so I comfortably fit into the free plan of my relay of choice (smtp2go). Other than a handful of recipients, I have had no trouble sending email anywhere, and I have much more control over what I receive and how by self-hosting. Even if I had to use a relay for most of my outgoing mail, I'd still self-host my e-mail, because it gives me a whole lot more control and privacy. With that said, way back when I started self-hosting, I also had to use a relay for some recipients, for the exact same reason: them using allow-lists. Back then it was my university, now it's my kids' school (a curious coincidence, I guess). There were always hosts that played a different game. Sure, they've concentrated into Google and Microsoft by now, but I can still send e-mail into those systems, even if through a relay, so self-hosting is still possible, and still gives you plenty of benefits.

I've been self-hosting my email for the past... almost 30 years. Today, I think it is easier to do so than 30 years ago. There's more to set up, but those are well documented, and with solutions like nixos-mailserver, mostly automated away. But the tools got better too! My setup catches a lot more spam now than it did a few decades ago, using a fraction of the resources, and tweaking my spam filters and other properties of the setup are considerably easier too.