In the Ceph case ceph-ansible has playbooks to correctly handle the
scale down of different ceph services, e.g. delete a monitor [1] or
delete an OSD [2]. The process would be to generate a ceph-ansible
inventory, e.g. the same way we do when we scale up [3], and then
execute one of those playbooks with that inventory. Examples of
running these playbook are in Seb's blog [4].

This would be a great feature to have because if you don't tell the
Ceph cluster that a node is not part of it anymore, then it will want
to find it and not be happy if you just delete the node. It's better
to tell the ceph cluster not to worry about a particular node anymore
by running one of these playbooks before the node is deleted.