Wow. I found that surprising when I first read it, but some research into the Linux world has shown this to be true. (Apparently, both RedHat and Ubuntu stopped doing this around 2020. What a shame.) This is interesting because by default, cron and systemd still sends (or tries to send) emails as notifications.
Both Debian and FreeBSD still appear to do so, so that’s good news.
I was also thinking of Alpine (you mentioned containers) and other lightweight, cloud-y base distros. But apparently Ubuntu hasn’t included an MTA by default for a decade or more.
Thanks everyone for the discussion and thoughtful input. 
I’m updating the DEP now. It finally feels coherent: all the details seem to fit together without obvious compromises or rough edges. (At least in my head. Of course, we’ll probably uncover other design issues during implementation. But for the first time, I don’t know where there be dragons.)
If we’re still thinking EMAIL_PROVIDERS could land in 6.1, I’ll try to sync up with Jacob R and dive into implementation next week.
With my 6.1 release manager hat on, the only items with higher precedence than this right now are the flood of security reports. I know Natalia is very keen to advance this as well. So short answer, “yes!”