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It depends on the type of attribute you have setup in the AD Connect provisioning, AFAIK the attributes that are synced to the user object in Azure AD will be retained, unless you manually remove the mapping from the user object from on-prem AD.
Yes you are correct skip deletion of user accounts will soft delete the object in Azure AD and will remove the object incl the attributes.
Please check the attributes that you have configured and it will give you detailed information.
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