supervise-daemon is still experimental and has bad defaults.
Users can enable supervise-daemon, if they want to, simply by declaring
`supervisor=supervise-daemon` in /etc/conf.d/smartd.

See my mail from Tue, 15 Dec 2020 15:33:16 +0100 in alpine-devel
mailing-list in topic "Use of supervise-daemon in Alpine".
(I cannot simply reference it from ML archive because it's broken.)