Google allows one to sign in with more than one account at the same time. For example, from the Gmail web interface, clicking on your own avatar in the top right corner shows your current account and an "Add account" button (along with "Sign out").
My wife did some work for a school last year and added her account from their organisation to her Gmail. She no longer works with them and we presume they have deleted her account from their organisation. Now, she would like to remove that account from the list of accounts shown in the account dropdown.
I have found some instructions that say you can remove things from that list by signing in to the alternate account using that list, and then using the "Sign out" button to remove it. However, that doesn't work in her case because she cannot sign in to do so (we get an error message which presumably indicates her account there no longer exists).
How can she dissociate her personal account from the external organisation account?