On Bitbucket, it would be good if there was a warning message on the SSH key restrictions page. Located at Administration > Keys and tokens:

Specifically when selecting ED25519 as forbidden. 

The objective is to warn that from Bamboo 9.0 onwards, the default SSH key created for Bitbucket integration uses ED25519 as the default SSH key. So if this key type is forbidden, the integration will not work correctly.