The warning looks something like this:
[ok] Running d6_term_node_revision:11
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
[warning] array_flip(): Can only flip STRING and INTEGER values! EntityStorageBase.php:227
The cause is something like this. In the EntityRevision destination the following code resolves $entity_id to NULL.
$entity_id = $row->getDestinationProperty($this->getKey('id'));
$entity = $this->storage->load($entity_id);
EntityStorageBase has no validation on its input value so that NULL gets passed in an array to loadMultiple which does an array flip and we get the warning.
Now all of that technical bit is pretty straight forward, how we got there is a bit more confusing to me because the d6_term_node migration seems to be set up to skip empty values resolved for the nid field but that doesn't seem to be happening for some reason.
Needs work
9.5
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Its hard for me to confirm this at this point. I actually had several more fixes that where needed on top of this.
My guess is that it was either in a field of the row was skipped or something leading to and destination id that wasn't valid?