When hosts are removed from an Ops Manager group they go into the Deleted Hosts lists, now under the 'More' drop down in the Deployment UI.
I cant find a reference to this in the 3.4 docs.
When hosts are in the deleted hosts list they are effectively blacklisted and cant be re-used. I can't find a reference to deleted hosts in the 3.4 documentaton.
I found this Ops Manager 1.6 docs link which mentions Deleted Hosts, but the equivalent page isn't available in the 3.4 docs.