Comment 11 for bug 1956787

Still digging into some history, found this patch in the package:

debian/patches/27-systemd-enable-with-systemctl-statd.patch
Description: Let sysadmins enable/disable statd services
 As the admin was able to control under upstart the statd services with
 NEED_STATD in default conffiles, mirror this funcationality under systemd
 by letting the user systemctl enable/disable statd services.
Author: Didier Roche <email address hidden>
Bug-Ubuntu: https://launchpad.net/bugs/1428486

Which removes the Wants for rpc-statd in nfs-server.service:
--- a/systemd/nfs-server.service
+++ b/systemd/nfs-server.service
@@ -4,8 +4,7 @@ DefaultDependencies=no
 Requires= network.target proc-fs-nfsd.mount
 Requires= nfs-mountd.service
 Wants=rpcbind.socket
-Wants=rpc-statd.service nfs-idmapd.service
-Wants=rpc-statd-notify.service
+Wants=nfs-idmapd.service

Apparently starting statd or not was controlled by a NEED_STATD var in /etc/default, and that is gone. To not always start statd (because it's not needed in nfsv4 I guess), they removed it from Wants, and let it be controlled via its unit file. That patch is from 2015.

Looks like the final paragraph of your comment #8 was right on the spot.