From the description you provided (@odata.context), It seems you have used delta Groups instead of delta Users.
To address your issue I have created 5 users in AAD and added them to a Group. Based on my testing I found that after deleting users from AAD I am able to get them in response of this GET /users/delta delta query i.e., annotation: @removed with value of "reason": "changed".
Please note that deleting user from Azure active directory does not permanently delete user.
However you can refer these documentation1, documentation2 to permanently delete users using MS Graph & AAD respectively. Post following this, I'm able to get annotation: @removed with value of "reason": "deleted" which indicates users have been deleted permanently from AAD.
To check the status of users deleted from an AAD Group use this GET /groups/delta delta query, You will get them under "members@Gabriel ". Please refer to this below screenshot.
Hope this helps.
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