The add-on does not collect any kind of data from users. That includes but is not limited to :
login credentials
users identifiers
diagram data (UML markup)
diagram image preview
any kind of data generated by the user, Confluence or any other add-on(s)
usage statistics
The add-on is fully compliant with Atlassian’s requirements for cloud applications listed here.
In short, all outgoing and incoming traffic is encrypted using TLS 1.2 and TLS 1.3 protocols.