Hmm, unfortunately, JIT only works for user provisioning, not for their removal or de-provisioning.
There’s no service, which would be checking the actual list of users in your Auth0, then comparing it with a list of users in GD and then deleting them also from the organization - if this is what you are after.
From the authentication point of view, there should be no issue. Once they are removed from the OIDC, they cannot be authenticated anymore.
If you are asking due to user limits(or any other concern), then I am afraid there is no out-of-the box solution. Such users would need to be removed manually via UI or API.