Disabled accounts
Disabled accounts are synchronized as well to Azure AD. Disabled accounts are common to represent resources in Exchange, for example conference rooms. The exception is users with a linked mailbox; as previously mentioned, these will never provision an account to Azure AD.
The assumption is that if a disabled user account is found, then we won't find another active account later and the object is provisioned to Azure AD with the userPrincipalName and sourceAnchor found. In case another active account will join to the same metaverse object, then its userPrincipalName and sourceAnchor will be used.
Reference:
https://learn.microsoft.com/en-us/azure/active-directory/hybrid/connect/concept-azure-ad-connect-sync-user-and-contacts