With Bionic, Sendmail would send email over IPv4 and IPv6, which I could see in logs and DMARC aggregate reports. Since upgrading to Focal, it's only attempting to send over IPv4, but I can receive email just fine over IPv4 or IPv6. I'm seeing that both with an upgrade and a fresh install.
What changed, then? Not my configuration (in the case of the upgrade, anyway), and I don't see anything relevant in the package changelogs between the 2 versions.
EDIT: On further testing, it looks like Sendmail in 20.04 isn't even trying to get IPv6 addresses for servers. If only an IPv6 address is available, I get a "host unknown" bounce.