The certbot-systemd-timer package is missing for Leap 15.4. It is present for
15.3 in
https://build.opensuse.org/package/show/openSUSE:Backports:SLE-15-SP3/certbot-systemd-timer.

The source for certbot on 15.3 and 15.4 are the same so we could submit the
same package for 15.4 - I've downloaded the rpm from 15.3 and am running it on
15.4 since yesterday without any issues (and one successful renew). I would do
it myself but have no idea how to do this for Leap, so it would be great if
someone could take care of this.

Thanks!