Partner Portals should include what other portals a user is assigned to under the Users and Teams.
When viewing your users and teams here: https://app.hubspot.com/settings/l/users
There should be a listing in the profile of all other portals this user is associated with. Naturally this would only be for portals and users that are partners as you wouldnt want this for a general portal (for example showing a user who is a freelancer and letting that portal know all the clients the freelancer works for).
Likewise, when removing an employee who left or has been terminated, you should be able to have the option of "remove from all client portals" with a list of id's/names.
Below is a sample mockup of these: