Control: severity -1 important nagios4/4.4.6-2 currently in unstable seems to build fine with gcc-10/10.2.0-18 on the builders [1], so I guess this was fixed? I'm downgrading the severity to let nagios4 migrate to testing.
Emmanuel Bourg [1] https://buildd.debian.org/status/package.php?p=nagios4&suite=unstable
