Currently, the entity reference for the host in a core is read-only, once the core entity has been created. Not sure why we've done that but it may well be necessary to change that setting. So let's see if and under which circumstances we can open that up.
Also, when deleting a host entity, the attached cores will be lost without any host assigned. There we should check during deletion, if cores are assigned and if so, ask for a different host before deletion.
Needs review
3.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Hmm, this is still disabled at the moment. I implemented a form alter to remove the #disabled property. AFAICS, that doesn't really have any implications, or are there any?
Please, if you want to contribute, then use the MR workflow, not patches. And if this is being implemented, then please address the full scope of the issue, not just the one part about the disabled host field.